Curtin appeals from his conviction and sentence for traveling across state lines with intent to engage in a sexual act with a minor, in violation of 18 U.S.C. § 2423(b), and of use of an interstate facility to attempt to persuade a minor to engage in sex, in violation of 18 U.S.C. § 2422(b). We have jurisdiction pursuant to 28 U.S.C. § 1291. We affirm in part, reverse in part, and remand for a new trial.
I.
On the afternoon of February 11, 2004, Las Vegas Metropolitan Police Department Detective Michael Castaneda was acting undercover on the Internet as a 14-year-old girl using the screen name “chris-tinel3.” Castaneda entered a chat channel labeled “ltgirlsexchat” and received an instant message from Curtin, who used the screen name “M-42SOCAL.”
The detective, as “Christy,” and Curtin “chatted” through instant messaging for approximately four hours. They exchanged photos early in the conversation. Castaneda sent Curtin a picture of a female police officer, taken when she was fourteen years old. Curtin said his name was “Kenny” and that he was forty-two years old, divorced, and living in Anaheim, California. He told Christy that he was planning to travel to Las Vegas on Friday, February 13 and invited her to go to a “Penn and Teller” show on Sunday, February 15. Christy agreed.
Curtin extensively discussed sex with Christy during this conversation, saying that he would love for her to “spend the night” after the show and hoped to “get a room.” Curtin told Christy “I want to make you happy.... If you were masturbating and fantasizing about sex, I’d love to have sex with you.” He added that they “could just make out or I could just give you oral sex or we could just fool around.” Finally, Curtin made plans to meet Christy in the bowling alley of a Las Vegas casino at 2:00 p.m. on Sunday, February 15. At the end of the conversation, Curtin asked Christy to try sleeping naked that night, and to “imagine my face moving between your legs and licking you. Imagine my tongue penetrating you.”
The next day, Curtin sent Christy an email message saying “I can’t tell you how much I’m looking forward to Sunday. We’re going to have a great time.” The detective and Curtin later that day had another “chat” during which Curtin continued to make explicit references to having sex with Christy. Curtin concluded the “chat” by confirming their meeting and telling Christy he would introduce her to Penn and Teller as his niece, adding, “Let’s not get caught, ever.”
On that Sunday, the police officer whose picture was sent to Curtin waited in the bowling alley as a decoy, dressed in the clothes that Christy indicated she would be wearing. Eight to ten other law enforcement officers were also present. Curtin entered the bowling alley at 1:45 p.m. and walked towards the area where the decoy officer was sitting. He walked past her and then turned and walked past her again, looking at her each time. Curtin then left the area where the decoy was sitting and went to the back of the bowling alley, where he used his personal digital assistant. At the request of law enforcement officers, a casino security guard ap*1088proached Curtin and asked for identification. Curtin showed the guard a United States passport and subsequently left the bowling alley area of the casino.
Curtin reentered the bowling alley approximately 2:05 p.m. He looked around and again walked to the area where the decoy officer was sitting. After less than a minute, he moved even closer to her, looking in her direction the entire time. He stopped behind the officer and she turned and said “hi” to him. Whether he said “hi” in return is disputed.
He then left the bowling alley and started getting into a van, at which point law enforcement officers stopped and asked him for identification. He was detained by police and advised of his rights under Miranda. After Curtin waived these rights, he agreed to speak with the law enforcement officers. In a voluntary statement, he stated that he had traveled by car to Las Vegas for meetings. He explained that he was at the bowling alley to meet a female friend he had met on the internet. He admitted to using the screen name and email address used to contact Christy. Curtin explained that he often enters chat rooms and “role play[s]” as if he is engaged in “daddy/daughter” type conversations, and that he expected Christy to be a thirty- to forty-year-old woman pretending to be a girl.
Curtin was then arrested by the Las Vegas police. Upon searching his van and hotel room, police seized his digital assistant and laptop computer. The digital assistant contained over 140 stories about adults having sex with children. The laptop contained a list of chat channels that Curtin had accessed in the past, as well as pictures of girls whose names matched some of those in his “chat” list.
Curtin was indicted on one count of travel with intent to engage in a sexual act with a juvenile, in violation of 18 U.S.C. § 2423(b), and one count of coercion and enticement, in violation of 18 U.S.C. § 2422(b). Curtin filed two motions in limine asking the district court to exclude the stories found on his digital assistant from evidence. The district court denied these motions in limine. On the second day of trial, the government offered two of the stories, “My Little Sister” and “Love for the World,” to show modus operandi, intent, preparation, and knowledge. They were admitted over Curtin’s objection. The engineer who extracted the stories from the digital assistant testified that both stories were about a father having sex with his young daughter and the daughter’s enjoyment of the experience. However, when the government sought to introduce a third story, “Melanie’s Busy Day,” the district court stopped the questioning. The court allowed the government to ask general questions without admitting the stories, such as whether they all related to sex between a minor and an adult. However, recognizing the highly prejudicial nature of the stories, the court held that the story could be entered into evidence only if it tied into Curtin’s intent, knowledge, preparation, or modus operan-di.
The government then asked the court to make a preliminary legal determination about the admissibility of the remaining stories. The government argued that “Melanie’s Busy Day” was admissible to show general intent, modus operandi, preparation, and knowledge because it had language similar to that used by Curtin in his email to Christy, namely, language concerning oral sex and a child masturbating. The government argued that “Missing Big Brother,” which discussed how the adult did not want to hurt the child during sex, was also admissible for intent, modus oper-andi, preparation, and knowledge. The government made similar arguments with regard to seventeen other stories, with titles such as “I’m Being Molested,” “The *1089Good Girl,” “A Relative Interest,” “Restrictions,” “Teaching the Kids,” and “Mommy Juice.”
The following morning, Curtin renewed his objection to the admission of the stories, arguing that they were highly prejudicial and were being admitted to show propensity. The district court held that if the government could cite a part of the story that related to one of the permissible purposes under Federal Rule of Evidence Rule 404(b), then the court would admit the entire story to show general intent.
The district court admitted to being unable to read the stories because they were so disturbing. “I thought about this problem overnight because I want to address the relative overwhelming prejudice versus the purpose. I read the first story and a little bit of the second. That’s as far as I could get, which is confirming [defense counsel’s] statement. It has a tendency to overwhelm you and overwhelm the jury.” However, after more argument, the district court agreed to give a limiting instruction and to admit five of the stories: “My Little Sister” (which involved incest and the impregnation of a nine year-old girl), “Love for the World” (which involved incest), “Restrictions” (same), “Daddy’s Lessons” (same), and “Melanie’s Busy Day” (which involved an eleven-year-old girl initiating sex with, among others, her father and her teacher).
The limiting instruction given was as follows:
A person cannot be charged nor convicted of literature that they read or that they possess. That’s why I’m giving you the instruction.
But the Government has the obligation to prove, beyond a reasonable doubt, that the defendant had the wrongful intent. They may offer possession of such literature to show that....
You may take this kind of evidence on the question of whether the defendant actually possessed the intent.
You may also take it on the additional questions which go to the question of intent, whether he practiced in this alleged conduct methodology consistent with literature that he had or tending to show that he prepared to commit the acts or that he had knowledge, that is, of how to commit the act or that the act was illegal....
So, for those four reasons, only, the Government is offering to show that the defendant possessed this literature; intent, method, preparation, and knowledge. And you may only take it for that purpose.
Again, you have a constitutional right. You have that right. You would want to protect the defendant’s right to possess any kind of literature and to read it or not read it.
You must not allow this kind of evidence to bias you, generally, against the defendant on the ultimate question of guilt or innocence. You must not do that.
II.
Curtin contends that the five stories were inadmissible character evidence, introduced to show propensity in violation of Rule 404(b) of the Federal Rules of Evidence. The government responds that the stories fall outside the parameters of Rule 404(b) because they are inextricably intertwined with the charged crimes. Alternatively, the government argues that the stories were properly admitted under Rule 404(b) to prove Curtin’s intent.
A.
The government argues that the admitted stories were “inextricably intertwined” with the facts giving rise to the indictment against Curtin and therefore not “other acts” evidence within the scope *1090of Rule 404(b). We review de novo the question of whether the evidence was within the scope of Rule 404(b). See United States v. DeGeorge, 380 F.3d 1203, 1219 (9th Cir.2004).
Two categories of evidence may be considered “inextricably intertwined” with a charged offense and therefore admitted without regard to Rule 404(b). First, evidence of prior acts may be admitted if the evidence “constitutes a part of the transaction that serves as the basis for the criminal charge.” DeGeorge, 380 F.3d at 1220, quoting United States v. Vizcarra-Martinez, 66 F.3d 1006, 1012 (9th Cir.1995). In United States v. Montgomery, 384 F.3d 1050, 1062(9th Cir.2004), we held that a government report detailing individual fraudulent acts was “inextricably intertwined” with the underlying conspiracy charge because the acts themselves comprised the conspiracy. Therefore, the report was admissible without regard to Rule 404(b). Similarly, in United States v. Lillard, 354 F.3d 850, 854 (9th Cir.2003), we concluded that the defendant’s theft of cocaine from a shipment, which was itself the basis for the conspiracy, was “inextricably intertwined” with the conspiracy charge.
Second, prior act evidence may be admitted without regard to Rule 404(b) “when it [is] necessary to do so in order to permit the prosecutor to offer a coherent and comprehensible story regarding the commission of the crime.” DeGeorge, 380 F.3d at 1220, quoting Vizcarra-Martinez, 66 F.3d at 1012-13. “A jury is entitled to know the circumstances and background of a criminal charge. It cannot be expected to make its decision in a void — without knowledge of the time, place, and circumstances of the acts which form the basis of the charge.” United States v. Daly, 974 F.2d 1215, 1217(9th Cir.1992), quoting United States v. Moore, 735 F.2d 289, 292 (8th Cir.1984). In United States v. Beck-man, 298 F.3d 788, 794 (9th Cir.2002), the defendant was charged with importing over 1,500 pounds of marijuana. His defense was that he was tricked into transporting the marijuana by the chief government cooperating witness. This witness testified at length about prior drug runs that Beckman had made on his behalf. We held that the witness’s testimony was “inextricably intertwined” with the charged offense, because it was “intended to establish [the witness’s] relationship to Beckman [and] to show that the relationship was ongoing....” Id.
The fact that Curtin was in possession of stories detailing sex with children at the time he went to meet “Christy” is insufficient to support the introduction of the stories without regard to Rule 404(b). Curtin was charged with traveling across state lines with intent to engage in a sexual act with a minor, and with attempting to persuade a minor to engage in sex. The possession or content of the stories was not a part of the “transaction” that led to the present charges. Second, the admission of the stories was not required for the prosecution to “offer a coherent story.” The prosecution would have had no difficulty in presenting all relevant evidence, including the “chat” conversations, Cur-tin’s admission that he was “M-42S0CAL,” and Curtin’s behavior at the meeting place, without the admission of the stories. Thus, the stories were not “inextricably intertwined” with the charged crimes.
B.
In the alternative, the government contends that the stories are admissible under Rule 404(b) regardless of whether they are inextricably intertwined with the crime. We review a district court’s admission of evidence under Rule 404(b) for an abuse of discretion. United States v. Romero, 282 F.3d 683, 688 (9th Cir.2002).
*1091We use a four-part test to determine whether evidence is admissible under Rule 404(b). United States v. Spillone, 879 F.2d 514, 518 (9th Cir.1989). First, “there must be sufficient evidence to support the jury’s finding that the defendant committed the other [act].” Id. Second, “the other [act] must not be too remote [in time].” Id. at 519. Third, when admitted to prove intent, “the prior act must be similar.” Id. Finally, “the prior act must be introduced in order to prove a material element of the case.” Id. The main issue here lies in the third element: whether there is similarity between the possession of the stories and the crime with which Curtin is charged.
Curtin objected to admission of the stories in the district court, relying on Guam v. Shymanovitz, 157 F.3d 1154(9th Cir. 1998) (as amended). Shymanovitz was a middle-school guidance counselor who was charged with sexually and physically abusing several of the boys under his supervision. Id. at 1155. Prior to trial, the government sought to introduce two magazine articles from sexually explicit magazines found in Shymanovitz’s home, arguing that they were relevant to establishing intent. The articles were both presumably fictional stories. One depicted sex between a father and son, while the other depicted sex between a priest and a child. The court deferred ruling on the motion.
At trial, the court permitted a police officer to testify that at Shymanovitz’s house she seized, among other things, condoms, surgical gloves, children’s underwear, and sexually explicit magazines. The officer testified in great detail, over defense counsel’s objections, about the contents of four of these magazines. She told the jury that the magazines contained explicit photographs of homosexual sex and described the photographs and the contents of the two articles in detail. The two articles were subsequently entered into evidence. Id. at 1155.
Both Shymanovitz and this appeal address whether the reading material at issue was admissible under Rule 404(b). We concluded in Shymanovitz that the magazine articles failed to constitute a Rule 404(b) “bad act.” “[P]ossession of lawful reading material is simply not the type of conduct contemplated by Rule 404(b).” Id. at 1159. Additionally, we held that possession of lawful reading material was not similar to actual criminal conduct, thus failing the third criterion of the Spillone test. .“[T]here is simply no doubt that a wide gulf separates the act of possessing written descriptions or stories about criminal conduct from the act of committing the offenses described.” Id.1
*1092On the issue of similarity, Shymanovitz conforms with the rest of our case law. In Vizcarra-Martinez, the defendant was charged with conspiracy to possess a chemical with reason to believe it would be used to make methamphetamine. 66 F.3d at 1009. The district court admitted evidence that Vizcarra-Martinez was in possession of a personal-use amount of methamphetamine at the time of his arrest. We reversed. “We simply cannot assume ... that Vizcarra-Martinez’s use of methamphetamine tended to prove that he knew that the chemical in his possession would be used in the methamphetamine manufacturing process.” Vizcarra-Mar-tinez, 66 F.3d at 1015. The cases in which we have allowed Rule 404(b) evidence show a much stronger connection between the “other act,” which is often a crime in itself, and the charged crime. In United States v. Vo, 413 F.3d 1010, 1018-19 (9th Cir.2005), where the defendant was charged with possession of methamphetamine with intent to distribute, we allowed the admission of a prior conviction for drug selling. “Vo’s prior conviction was evidence of his knowledge of drug trafficking and distribution in general. The conviction tended to show that Vo was familiar with distribution of illegal drugs and that his actions in this case were not an accident or a mistake.” Id. at 1019.
Shymanovitz does recognize a narrow exception for “modus operandi” evidence: evidence involving acts by the defendant that are “so nearly identical in method as to ear-mark the charged offense the handiwork of the accused and that are so unusual and distinctive as to be like a signature.” Id. at 1159 n. 9 (internal punctuation and citation omitted). “[U]se of modus operandi evidence is rare, and the similarities must be specific and detailed and clearly set the particular offense apart from the general body of such offenses.” Id.
The evidence in this case is not sufficiently detailed and specific to fall within the exception. The stories did not describe specific methods of committing the first offense with which Curtin was charged: traveling across state borders to have sex with a minor. Rather, the stories mostly involved explicit descriptions of incest. Thus, the stories do not reveal a relevant modus operandi to commit the charged crime, and are inadmissible. The issue is closer with regard to the charge of use of an interstate facility to attempt to persuade a minor to engage in sex. The government argued that the language in the stories was similar to the language used by Curtin in his “chat” conversations. We conclude that Shymanovitz forecloses our acceptance of the government’s position. The conduct with which Curtin was charged, and the language that Curtin used, can “hardly be construed as either distinctive or remarkable in the universe of sexual offenses against minors.” Id. Compare United States v. Dhingra, 371 F.3d 557, 566-67 (9th Cir.2004) (as amended) (allowing introduction of Rule 404(b) *1093modus operandi evidence where “[i]n both instances, Dhingra contacted a minor under the age of 18 years over [instant messenger] for the purpose of soliciting sexual activity, arranged to meet, and at the meeting attempted to engage in sexual activity by persuasion and coercion.”). The similarities between the stories and Curtin’s conduct were not distinctive or unusual enough to render the stories admissible as modus operandi evidence.2
The government relies on United States v. Allen, 341 F.3d 870 (9th Cir.2003), to distinguish Shymanovitz. In Allen, the defendants were charged with violating federally protected rights on the basis of race and religion. We allowed the introduction of “skinhead and white supremacist evidence,” including color photographs of the defendant’s tattoos (e.g., swastikas), Nazi-related literature, group photographs including some of the defendants (e.g., in “Heil Hitler” poses and standing before a swastika that they later set on fire), and other skinhead paraphernalia. Id. at 885-86. We distinguished Shymanovitz: “Key to our reasoning [in Shymanovitz ] was the fact that the testimony ... was not relevant to proving any of the elements of the crime for which the defendant was convicted.... ” Id. at 887 n. 25. The government thus contends that, when read together, “Shymanovitz and Allen stand for the principle that relevant literature is admissible to prove intent.”
However, the government’s comparison to Allen does not save the day. First, many of the items admitted into evidence in Allen involved more than the possession of reading material. The defendants were active participants in “Heil Hitler” poses and had posed with and burned a large swastika. Second, unlike in Shymanovitz, there is no indication that the reading material in Allen was fictional. Third, the evidence at issue in this case, and in Shy-*1094manovitz, was extremely prejudicial. Finally, it is possible that the evidence in Allen would have been admissible under the “inextricably intertwined” exception outlined above. Our holding here is controlled by Shymanovitz. The district court committed an abuse of discretion .by admitting the stories.
C.
The government fails to argue harmless error and thus the issue is ordinarily waived. See United States v. Varela-Rivera, 279 F.3d 1174, 1180 (9th Cir. 2002). We may, however, consider harmlessness sua sponte in “those unusual cases in which the harmlessness of any error is clear beyond serious debate and further proceedings are certain to replicate the original result.” United States v. Gonzalez-Flores, 418 F.3d 1093, 1100 (9th Cir.2005). “[T]he court’s certainty as to the harmlessness of the error ... is of particular importance.” Id. at 1101 (citations omitted). “[E]rror is harmless if we can say with fair assurance that it did not have a substantial effect, injurious to the defendant, on the jury’s decision-making process.” Arnold v. Runnels, 421 F.3d 859, 867 (9th Cir.2005) (citations omitted).
Here, we do not have the requisite level of certainty that the error was harmless to consider the issue sua sponte. Most of the sexual activity described in the stories admitted from Curtin’s PDA was incestuous, a particularly prejudicial taboo. The admitted stories were so highly disturbing that even the district court judge stated he was unable to read them. The issue of harmless error is waived.
III.
Curtin contends in his reply brief that the district court abused its discretion pursuant to Rule 403 by admitting the stories because the probative value of the stories is substantially outweighed by the danger of unfair prejudice to Curtin. However, Curtin did not specifically and distinctly make a Rule 403 argument in his opening brief. “[0]n appeal, arguments not raised by a party in its opening brief are deemed waived.” Smith v. Marsh, 194 F.3d 1045, 1052 (9th Cir.1999) (citation omitted); see also Greenwood v. FAA, 28 F.3d 971, 977 (9th Cir.1994). The argument is waived. In any event, for the same reason we cannot say that the error was harmless, we cannot say that the admission of the materials on Curtin’s PDA was not unduly prejudicial.
IY.
Curtin argues that the government’s failure to request a copy of the surveillance video from the casino violated due process and entitled him to a spoliation instruction. We disagree. The government’s duty to preserve evidence arises when the evidence “possesses] an exculpatory value that was apparent before the evidence was destroyed, and[is] of such a nature that the defendant would be unable to obtain comparable evidence by other reasonably available means.” California v. Trombetta, 467 U.S. 479, 489, 104 S.Ct. 2528, 81 L.Ed.2d 413 (1984). “Unless a criminal defendant can show bad faith on the part of the police, failure to preserve potentially useful evidence does not constitute a denial of due process of law.” Arizona v. Youngblood, 488 U.S. 51, 58, 109 S.Ct. 333, 102 L.Ed.2d 281 (1988). As Curtin failed to show that the video had any exculpatory value, the district court did not abuse its discretion in declining Curtin’s request for a spoliation instruction.
Curtin also argues that the district court failed to instruct adequately on what constitutes a substantial step. The district court instructed in accord with the model jury instructions that “mere prepa*1095ration was not a substantial step.” See Model Crim. Jury Instr. 9th Cir. § 5.3 (2003). This fairly and adequately covered the issue. See United States v. Echeverry, 759 F.2d 1451, 1455 (9th Cir.1985) (“So long as the instructions fairly and adequately cover the issues presented, the judge’s formulation of those instructions or choice of language is a matter of discretion.”).
AFFIRMED IN PART; REVERSED IN PART; REMANDED.
. The dissent argues that this case differs from Shymanovitz because the crimes at issue in that case did not involve subjective intent. But even if Shymanovitz involves slightly different facts, its legal rule does not distinguish between specific intent crimes and other crimes. "Under the government’s theory, the case against an accused child molester would be stronger if he owned a copy of Nabokov's Lolita, and any murder defendant would be unfortunate to have in his possession a collection of Agatha Christie mysteries.... ” Shyma-novitz, 157 F.3d at 1159. Put simply, Shyma-novitz is replete with language that dictates the outcome of this appeal. We may not ignore its controlling effect on this case. See Barapind v. Enomoto, 400 F.3d 744, 751 n. 8 (9th Cir.2005) (en banc) (per curiam).
The dissent's discussion of cases like United States v. McCollum, 732 F.2d 1419, 1425 (9th Cir. 1984), which involve the admission of similar prior criminal convictions to prove intent, is equally off point. In McCollum, we allowed admission of a prior conviction for armed robbery to negate the defendant’s defense of acting under hypnosis. The relationship between the prior crime and the act charged is obviously far closer in McCollum’s case than it is in Curtin's. Curtin's case involves otherwise lawful reading material that is not similar to the crime with which he is *1092being charged. Essentially, the government here seeks to introduce possession of material that describes one type of criminal conduct, namely incest with minors, to prove intent to commit another type of criminal conduct, namely crossing state borders to have sex with a minor. This is impermissible under our caselaw.
The dissent also argues at length that Cur-tin's case is different because Curtin was actually in possession of the stories at the time of his arrest. However, the government never argued relevance based on physical possession. In any event, this is a fallacious distinction because the stories were saved as documents to Curtin's handheld computer. In the absence of any evidence to the contrary, we should not infer that Curtin planned to "use” the stories or had the stories in mind simply because he was carrying his computer at the time of his arrest.
. Contrary to the dissent’s assertion, we have not "made relevant literature off limits in the Ninth Circuit as a matter of law.” Nor do we "hamstring!] the capability of the rule of law to cope in this Circuit with adults who see children as sexual prey.” As pointed out above, in this case the prosecution may rely on Curtin’s presence in a chat room called “Itgirlsexchat,” the "chat” conversations themselves, Curtin’s admission that he was "M-42SOCAL,” and Curtin’s behavior at the meeting place.
In terms of otherwise lawfully-possessed literature, modus operandi evidence and inextricably intertwined evidence remain admissible. Here, however, the jury was asked “to infer from behavior on one occasion something about the nature of a person and then to infer from that how the person probably would have behaved on another occasion when the only connection between the two occasions is that thefjury] believes that people of a certain type would act the same way both times.” 1 Stephen A. Saltzburg et al., Federal Rules of Evidence Manual § 404.02[9](8th ed.2002). Without more of a connection between the literature and the accused crimes, the literature is no more than character evidence introduced to show propensity, and highly prejudicial character evidence at that.
The dissent argues at length that the evidence was necessary to refute Curtin’s testimony and "aggressive defense” of his intent to meet an adult, and attempts to distinguish Shymanovitz because Curtin actually testified. However, the stories were not offered for impeachment, on which we express no opinion. Nor do we believe that the stories were necessary given the strength of other evidence introduced by the government. In any event, evidence is not admissible solely because it may be necessary or helpful. Wholly apart from its prejudicial effect, we have held that "[t]he mere possession of reading material that describes a particular type of activity makes it neither more nor less likely that a defendant would intentionally engage in the conduct described and thus fails to meet the test of relevancy under Rule 401.” Shymano-vitz, 157 F.3d at 1158. Here, the materials in Curtin's possession told stories of incestuous sexual acts that were different from those he was accused of intending to perpetrate. They were offered as substantive evidence of guilt, and we simply hold that their admission in this case was an abuse of discretion.