The remaining five team members become increasingly panicky about finding out what the secret is. Will suggests that Finn (who is being held separately from them) might know something helpful. The group decides to create a distraction, giving Georgina time to slip away to the closet where Finn is tied up. The plan to distract Callum is only partially successful and Will ends up being the one with an opportunity to get out of the dining room. He hurries to find Finn. Finn manages to tell Will that he is a private detective and gives him the small notebook he had been writing in earlier in the evening. Before Will can learn more, Callum appears, shoots Finn, and turns the gun on Will.

In a July 1999 flashback, Will, Lily, and Sophie sneak into Professor Marlowe’s study, looking for materials that could help Lily in her project of designing a tool to detect traits associated with psychopathy. They do find relevant materials and continue to discuss the macabre nature of Marlowe researching psychopaths after his wife was murdered by a serial killer. Will suggests that Lily name her test “The Wasp Trap.” Later, they discover Dominic has burned the wasps’ nest, claiming one stung Eve (even though Sophie previously made him promise not to punish the wasps after the attack on Georgina). After the others leave, Dominic taunts Will about not making a move on Sophie, warning he will get “sloppy seconds.” Will feels angry and jealous.

From upstairs, Mia hears a second gunshot; she creeps to the top of the stairs, from where she can faintly hear Amber and Callum speaking. Based on their conversation, Mia deduces that Finn is dead and that Amber and Callum may be in a romantic relationship. Finn’s death is upsetting to Mia because his presence in the house is connected to her. The narrative goes on to explain events that have occurred prior to the dinner party. When Olivia disappeared in March 2023, Mia could not accept that her sister had run away. Obsessed with finding out what really happened, Mia decided to hire a private detective to investigate. She sold a bracelet Olivia had inherited and used the money to pay Finn. At this point in the narrative, it is not clear why Finn was at the dinner party.

Mia feels guilty about Finn’s death and is also terrified and desperate to escape from the house. Because she has the backdoor key, she has the ability to get out, but only if she can make it downstairs and to the door without being detected. Since Amber and Callum are patrolling the main floor, she sees no way of doing so.

After killing Finn, Callum asks if Rohan accepted the ring and is surprised to learn that he did not. Will gives the ring back to Callum. Before returning to the dining room and joining the others, Will also sees Callum and Amber exchange a passionate kiss. Will tells the group Finn was a private detective. Georgina admits she knew this information and that Finn was investigating Olivia’s disappearance. She explains to the group that Mia initially hired Finn of her own initiative; when Georgina found out, she was impressed by her daughter’s ingenuity.

Georgina provides more context into the motivation behind the dinner party. After working together for months, Finn and Mia came to her and presented their theory that one of the team members from the summer of 1999 might be connected to Olivia’s disappearance. Finn had developed this theory after meeting with Professor Marlowe, although Georgina is vague about what happened during this meeting. Georgina agreed to Finn and Mia’s suggestion of throwing a dinner party and inviting the team members as well as Finn so that the latter could observe them and try to find out information related to Olivia. The story of Finn being Marlowe’s assistant was invented to justify his presence. Georgina adds that even Theo did not know of this plan between her, Finn, and Mia—he believed (like the others) that Finn was Marlowe’s former assistant.

While hiding upstairs, Mia thinks back to past events. She recalls how Olivia was agitated the weekend before she vanished. Olivia only gave her sister very limited information but made comments about needing to talk to the professor and expressing regret that she had “gone in there” (232). At the time, Mia shrugged off the strange conversation but after Olivia vanished, she was haunted by the exchange. Once Mia began working with Finn, she told him about this conversation and her inference that Olivia was referring to Professor Marlowe (both girls are aware that their parents met while working with him).

After months of delay, Finn finally reported to Mia that he had been able to meet with Marlowe. Finn went to Thornwood, where he found Marlowe seriously ill and heavily sedated. Nonetheless, Finn tried to question Marlowe about whether Olivia had come to see him; he did not get a clear answer, but Marlowe became agitated and stated, “everything echoes” (the same phrase that Will had noted Finn and Callum using during the dinner party events). Finn was interrupted by a man who seemed to be Marlowe’s nurse; this man forced Finn to leave and claimed no young girls had come to Thornwood. Nonetheless, Finn believed Olivia may have sought out the professor or another team member around the time of her disappearance. After this meeting, Finn asked Mia to meet with her parents. It was at this point that they told Georgina what had been happening and hatched the plan for the dinner party.

In the dining room, Georgina continues explaining the backstory to the dinner party. She tells her former colleagues what Finn shared with her: after inspecting Olivia’s laptop, an expert recovered Olivia’s deleted search history, revealing she had looked up all the dinner guests on Google and social media the week before she disappeared. The guests deny that Olivia ever contacted them or that they know anything about what happened to her.

Given Olivia’s efforts to investigate the former team members, and the insistence from Callum that someone knows a secret, Will wonders if Callum and Amber could be connected to Olivia’s disappearance. Callum enters, announcing it is time to vote again.

In an August 1999 flashback, Lily tests her psychopathy assessment on Will. The test involves viewing images and selecting emotional responses, answering personality questions and hypothetical scenarios, and a polygraph section. During the polygraph, Lily asks if Will loves Sophie. He admits yes. Lily encourages Will to confess his feelings for Sophie, revealing that Will and Sophie actually scored high for compatibility in the Butterfly Net assessment. Will is encouraged, although he still feels insecure that Sophie might prefer Dominic. Lily continues to administer the psychopathy assessment to other team members.

Back in the present day, voting begins again after no one confesses. Reluctantly, Will votes for Rohan, citing weak reasoning about his friendship with Dominic. Rohan votes for Will, claiming the novel Will is writing about that summer proves he must know all the secrets. Lily apologizes and votes for Will. Georgina casts the deciding vote, also choosing Will.

In an August 1999 flashback, the group celebrates completing Butterfly Net’s beta version. Sebastian gives them the weekend off. That evening, the six team members party along with Eve and Dominic, dancing and drinking heavily. Eventually, Will decides to confess his feelings for Sophie: since she is not outside, he goes inside to check her room. The door is closed and no one answers, so he decides to leave a love note on her pillow. Opening the door, he finds Dominic sitting on the bed and Sophie’s clothes on the floor. Sophie calls from the adjoining bathroom, and Dominic catches sight of Will, implying that he and Sophie are about to have sex.

Devastated, Will staggers into the hallway where Georgina finds him crying. She pulls him into her room for comfort, and they have sex. Immediately afterward, Will knows he has made a terrible mistake.

In the present-day narrative, Will confesses to sleeping with Georgina. This is the only secret he has been hiding. Georgina clarifies she and Theo had not discussed being exclusive when the incident occurred, and that sleeping with Will made her realize how much she liked Theo. Sophie reveals that the entire event was a misunderstanding: She never slept with Dominic. He came to her bedroom for a tarot reading and Sophie was in the bathroom performing her pre-reading ritual when Will arrived. In fact, Sophie had feelings for Will and subsequently tried to find him but was confused as to why he suddenly behaved coldly toward her.

Callum declares their secrets entertaining but not what he is looking for. He and Amber prepare to kill Will and Georgina. Will tells Sophie he still loves her and should have told her 25 years ago. Lily suddenly announces she is the one with the secret.

In a flashback to 1999, Lily is up late working alone. She has now processed the psychopathy test results. Most scores were unremarkable, but Dominic scored 83—clearly identifying him as a psychopath. Lily wrestles with ethical questions about what to do with that information. She decides to consult with Sophie and searches the house before going looking for her outside on the grounds of the estate.

In the dark woods near the lake, Lily hears Eve and Dominic arguing. She watches as Dominic chases after Eve; he has tried to sexually assault her and now tries to persuade Eve not to tell anyone. The two of them struggle and Dominic strikes Eve in the head with a tree branch. Lily springs forward as Eve collapses; she tackles Dominic and he falls face-first into shallow water, striking a rock. Lily kneels on his back, holding him underwater until he goes still. Wrestling with her conscience, she decides his psychopathy makes him too dangerous to save and leaves him in the water to drown.

Lily checks Eve, who is dead, and goes back to the house, leaving both bodies at the scene. She tells Professor Marlowe about what happened, although she implies that Dominic’s death was accidental. She also explains about the psychopathy assessment and Dominic’s exceptionally high score. Marlowe reassures Lily and tells her to go back to her room and not tell anyone what happened. He promises to deal with the situation.

In the present-day, Lily continues her confession. Sophie realizes that Dominic must have interpreted the results of the tarot card reading (which took place just before the events in the woods) as encouragement to pursue Eve. Lily explains how the next morning, Professor Marlowe abruptly shut down the project and sent everyone home separately. She never learned the details of how Marlowe concealed the deaths, but when he broke the news of Dominic’s drowning a week later, this confirmed his cover up. Nonetheless, Lily has been racked by guilt and fear ever since.

Georgina demands to know why Lily didn’t confess sooner; she points out that Lily also actively lied when Eve was discussed (Lily claimed that Eve was living in London, despite knowing that the latter had been dead for decades). Georgina claims that if Lily had revealed the secret sooner, Theo might still be alive. However, Callum declares this is still not the secret he is seeking. He leaves the dining room and returns with Dominic, who is somehow still alive.