The six former team members are astonished to see Dominic alive; they also note that he has aged badly and is now a shadow of his former confident and handsome self. Dominic and Callum bicker with one another, with Callum wondering if the secret even exists and Dominic insisting that it does (this confirms they have been working together). Dominic also appears to be in a romantic relationship with Amber which confuses Will (since he has also seen Amber and Callum kissing). Dominic is impatient and demands to know the secret; Callum explains that Rohan and Sophie have not yet revealed secrets.

Georgina interrupts, asking if Dominic knows where Olivia is. He says he does not but claims Olivia started everything. Will suggests Olivia might be the secret-holder. Confused, Dominic explains that Professor Marlowe told him it was one of the six team members. Lily demands to know how Dominic is still alive, and he agrees to explain.

In August 1999, Dominic regains consciousness face-down in the lake. He sees Eve’s body nearby and remembers killing her but cannot understand who struck him and pushed him into the water. Professor Marlowe appears, carrying Eve’s bicycle. He reveals that Lily pushed Dominic into the lake and believes she killed him. Marlowe seems strangely interested in Dominic and questions him about Eve, determining that it will be easy to cover up her death since no one is likely to come looking for her. Marlowe and Dominic load Eve’s body and her bicycle into the boat, which Dominic then rows out to the center of the lake. He sinks her body and the bike into the water.

Dominic confirms Eve’s body remains in the lake. He professes to have felt shock and deep remorse after the events of the night. He went back to the house and eventually Professor Marlowe came to his room and explained what was going to happen next. Marlowe was going to shut down the dating platform, send the team members home, and then turn his focus to researching psychopaths, with Dominic as his prime subject. Marlowe will conceal Dominic and protect him from facing justice, in exchange for Dominic being the subject of his experiments and research.

Dominic reveals information that he learned from Professor Marlowe over subsequent years: Marlowe had long been obsessed with psychopaths and had even suspected himself of being one. However, he was emotionally transformed when he met Barbara. After Barbara’s death, Marlowe conducted risky experiments to test what happened when psychopaths and sociopaths fell in love. This research led to his being fired, which is when he returned to England and began working on the dating platform project. However, “the incident with Eve happened and [Marlowe] flipped back” (313), once again being fixated on researching psychopaths. For over 20 years, Sebastian tested Dominic repeatedly, but none of the results matched the doctored score of 83. Unable to accept this, Sebastian became convinced Dominic was manipulating the tests. Sebastian kept Dominic prisoner not only through fear of prosecution but by promising to leave him Thornwood and his entire estate. Bitterly, Dominic tells Lily that her test ruined his life.

However, when Marlowe developed aggressive cancer, Dominic finally saw freedom approaching. He patiently nursed his uncle, awaiting his death. Dominic cuts off his story by commenting that Finn ruined everything.

The narrative flashes back to January 2024. When Finn came to Thornwood, Dominic had been feeling hopeful about his future. He was in a romantic relationship with Amber (whom he met when she participated in Marlowe’s research) and anticipated that he would soon inherit everything and start a new life with her. Dominic was startled when he interrupted Finn questioning Professor Marlowe and quickly rushed Finn away (letting Finn believe that he was merely Marlowe’s nurse).

The next day, Dominic questioned Marlowe, worried that someone might know about Eve’s death. Marlowe tells him that there is another secret beyond Eve’s murder—one that could destroy Dominic’s inheritance. Marlowe stated that one of the six team members possessed this secret but died before revealing who. Dominic began researching the six former team members and became obsessed with finding out which one of them knew the secret. Dominic was also hesitant to have the former team members find out that he was still alive.

On the day of Marlowe’s funeral, Dominic was alone at Thornwood when he saw Mia approaching the house. He immediately recognized her as Theo and Georgina’s daughter.

In the attic, Mia recalls visiting Thornwood on the day of the funeral. She had become convinced that the estate might hold information about Olivia’s fate and timed her arrival to sneak in while the house was empty but encountered Dominic. Like Finn, she presumed he was Sebastian’s nurse. However, Dominic quickly revealed that he knew Mia was there seeking information about her sister. Dominic told her that Olivia never visited the house but asked Mia to tell him more about what she has uncovered. Mia explained that she and Finn suspected that Olivia knew a secret related to the events of 1999, and that the two of them, along with Georgina, had planned a dinner party that would reunite all of the team members. Mia also told Dominic the date of the dinner party; he showed her around the estate and then she went home.

In the dining room, Dominic continues speaking with the former team members. He explains that, after Mia told him about the dinner party, he became determined to attend and enlisted Callum and Amber to help uncover the secret. They came up with the idea of the game as a way to lure out whoever knew the secret. Callum is helping in exchange for a percentage of Dominic’s inheritance. Will deduces that Dominic is not aware that Amber and Callum are still engaged in a romantic relationship.

Dominic explains that he intended to wait outside while Amber and Callum carried out the plan and obtained the secret; he only entered the house when he realized the plan was not going smoothly. Dominic becomes more agitated and decides he is going to kill them all unless someone immediately confesses the secret.

As Dominic prepares to execute everyone, Lily shouts for him to wait, claiming she can get him the secret. She proposes using her new lie-detector app, which is 99% accurate. She concedes her original psychopathy assessment may have been flawed, noting Dominic does display genuine emotions.

Dominic is skeptical but confers with Amber and agrees. Lily explains she needs the Internet restored in order for the app to work. Callum objects, fearing a trick, but Lily emphasizes she has kept silent about a murder for 25 years and simply wants to go home. Callum does not like this plan but Dominic orders him to restore the Internet and Callum complies. Will recognizes that the Internet is only being restored because Callum and Dominic now plan to kill all of them, no matter the outcome. The need to escape is more vital than ever.

Mia finds a small door in the walk-in closet and crawls through it. Inside the secret passage, she begins descending the narrow staircase, hopeful that it will lead her out of the house. Midway down, she notices a trunk at the far end of the passage but continues without investigating it. As she descends the stairs, Mia drops her phone and must continue in total darkness. When Mia estimates that she must be on the ground floor (but concealed behind the walls), she hears Callum muttering to himself as he turns the Internet back on. However, Mia realizes that when she comes to the bottom of the staircase, there does not appear to be any way to exit the passage.

While Callum is out of the room, Will tells Dominic that Callum and Amber are having an affair. Enraged, Dominic storms out to confront him, followed by an unarmed Amber. Alone, the five survivors debate escape. Sophie insists they will be killed anyway and must find a way out. Will embraces Sophie, confesses he still loves her, and they kiss. She discovers Finn’s notebook in his pocket (Finn had passed this to Will before his death, but Will forgot about it in the chaos of ensuing events).

Tucked inside the notebook,  they find a printout of a text message that was saved to a Drafts folder in March 2023 (around the time Olivia went missing). It is not clear who wrote it, or who it was intended for. The author of the message states that they have found something, but don’t know what to do with it, and have left it undisturbed for the time being. The author expresses uncertainty about sending the text. The message also references Claude (the family cat) having led the author somewhere (this language echoes what Mia recalls Olivia saying before she vanished). However, the team members speculate the message could be from Mia to Finn about a discovery related to Olivia.

As they discuss this, the three captors return. Amber denies being involved with Callum and Dominic chooses to believe her. He announces they will proceed with the lie-detector test.

From within the passage, Mia hears an argument between Dominic, Callum, and Amber. Realizing the Internet is on, she inserts her AirPods, which connect to her lost phone and activate her listening device in the dining room. She can now clearly hear the conversation happening there and she listens while Will reads the draft of the text message. Because of the context she has, Mia can immediately recognize that Olivia wrote the message (presumably to her boyfriend, Felix). Now that she has also uncovered the secret passage, Mia concludes that Olivia stumbled upon the passage and found something there (which can only be the trunk).

Mia hurries back up the staircase, retrieves candles from her parents’ bedroom, and uses them to provide light. She returns to the spot where the trunk is located midway down the passage and opens it.

Dominic locks down Lily’s phone connectivity and begins the lie-detector test. Lily goes first and easily passes, confirming she has no additional secrets and that she does not know who possesses the secret. Next is Rohan, who confesses he attempted to steal the Butterfly Net algorithm from Lily’s laptop in 1999. The app confirms he is telling the truth, but this is not the secret Dominic seeks.

The test moves to Georgina. She and Dominic begin arguing and she sneers that he was whining when he hauled Eve’s body into the boat. Dominic had told the group about hiding Eve’s body in the lake, but Georgina could only know this specific detail if she had been present and observing. Dominic finally has the information he has been seeking: Georgina was the one who witnessed him hiding Eve’s body. He admits that he has been lying to Callum and Amber: while Dominic didn’t know who possessed the secret, he did know what the secret was. Before Professor Marlowe died, he confided to Dominic that one of the six team members saw Dominic disposing of Eve’s body (this is the information that would jeopardize Dominic’s inheritance and send him to prison). Dominic became obsessed with finding out who saw him on that night.

Callum is furious that Dominic withheld this information. Dominic tries to rationalize that if Callum or Amber had revealed the secret to the team members, they would have had the ability to protect one another or invent confusing details. By withholding the secret, it would immediately be clear when the witness revealed it. Nonetheless, Callum is still angry, and he begins taunting Dominic, revealing that he and Amber have been sleeping together. Callum and Dominic lunge at one another and Callum accidentally drops the knife he has been wielding. Georgina and Amber both try to grab it. Georgina seizes the knife and fatally stabs her. As Dominic rushes to his dying girlfriend, Georgina grabs Amber’s shotgun and shoots Callum twice, killing him.

Will urges everyone to flee while they can. He pulls a reluctant Georgina, who wants to kill Dominic, from the dining room. The group races toward the cellar, since Georgina previously mentioned that it might be possible to escape through a small window located there. As they rush into the cellar, Dominic catches up to them and shoots Sophie in the leg.

Rohan drags Sophie through the cellar door and Will slams it shut. Lily, Georgina, Rohan and the wounded Sophie are now inside the cellar, but Will is trapped in the hallway with Dominic. Will tries to reason with the grief-stricken Dominic to buy time for the others. Dominic begins preparing to set the house on fire, and Will strikes him over the head with a wine bottle, knocking him unconscious. Will takes the shotgun and matches and descends into the cellar with the others.

Suddenly, a closet door opens and Mia emerges (she has followed the secret passage down to the cellar). Mia is clutching a notebook in her hand, and she abruptly accuses Georgina of killing Olivia.

The narrative flashes back to 1999, on the night of the party and Eve’s murder. Earlier that afternoon, Georgina had snuck into Lily’s room to view the results of Lily’s test for “psychopathy.” Because the test took time to run, and Lily had left it to run the results in her absence, Georgina was the first to see the results. She learned that she (not Dominic) was the one with the exceptionally high score, confirming that she was almost certainly a psychopath. Georgina was not bothered by this result, but she changed the name labels in the program and reprinted the results, giving the impression that Dominic was the psychopath. She switched her own score with Dominic’s result (which was within the normal range). Before leaving the room, Georgina also took the floppy disk that a copy of the assessment program was saved to.

Later that night, after having sex with Will, Georgina went outside and walked toward the lake. She caught side of Eve’s body and watched as Marlowe and Dominic discussed how to dispose of it. She hid, recorded their conversation, and stole the murder weapon (the tree branch, now stained with Eve’s blood). Georgina waited until Dominic had disposed of the body and returned to the house, and then she approached Professor Marlowe. This explains how Marlowe knew that Georgina was the one who witnessed the cover-up, whereas Dominic did not.

In the present-day narrative, Mia confronts Georgina. She explains that she found Olivia’s journal in the trunk in the hidden passage. Olivia’s journal describes that she in turn, found Georgina’s journal when she accidentally happened upon the secret passage. Olivia summarized the secrets she read in her mother’s journal: information about Georgina’s score on the test and Georgina witnessing the cover-up of Eve’s murder. Georgina used this information to blackmail Marlowe into giving her money.

After some attempts at denial, Georgina admits that Mia’s claims are true. Georgina hid her journals in the trunk in the secret passage because she thought she might need to blackmail Professor Marlowe again, and she felt confident that no one would ever find it. However, Olivia happened upon the secret passage while chasing after the family cat, found the journals, and confronted her mother. Determined to keep her secret, Georgina killed her own daughter. She explains to Mia that she went along with Mia’s efforts to hire Finn and throw the dinner party because this prevented her from looking guilty, and she felt secure that her secret would never come out. Georgina tells Mia that Theo is now dead as well, and they can move on with their lives; however, in order to conceal the secrets, they need to kill the remaining witnesses (Rohan, Will, Lily, and Dominic and Sophie, both of whom are already seriously injured).

As Georgina prepares to begin shooting, Will tackles her. They struggle and Georgina gains the upper hand and points the gun at Will. Mia strikes her mother with the bloody branch: This was concealed in the trunk along with the journal (Georgina kept it as collateral against Dominic and Professor Marlowe). Mia carried it down to the cellar along with Olivia’s notebook and she now uses it as a weapon. With Georgina unconscious, Rohan seizes the gun. While these events have been occurring, Sophie has been steadily bleeding out and is now close to death. Will is desperate to find a way to exit or call for help.

Mia has the key to the backdoor from when she snuck back into the house. She opens the door, Will runs outside and calls for help; an ambulance arrives to treat Sophie, and she recovers well. Georgina is arrested and the case immediately becomes sensationalized; Dominic is also arrested. Two weeks later, Will attends Theo and Olivia’s joint funeral. He meets Sophie, Lily, and Rohan at the crematorium. They see Mia with her grandparents. Lily reveals that Eve’s body has been recovered from the lake and Lily is unlikely to face charges even though she did not report the murder.

The survivors’ lives have transformed: Lily is starting a company to market her lie detector, Rohan is abandoning business dreams to focus on his family, and Will and Sophie are beginning a relationship. Will has started writing a book about the events. Rohan notes that Sophie is the only one who never revealed a secret and the group engages in dark humor about the terrible events of the dinner party.

Mia is living with her paternal grandparents, still adjusting to life with the elderly couple who fuss over her and cry frequently. The house is filled with photos of her father and Olivia, while all pictures of Georgina have been burned.

She recalls returning to the Notting Hill house and retrieving items from the secret passage, including a floppy disk labeled “Wasp Trap.” She acquired an old laptop compatible with the obsolete technology and discovered the disk contains Lily’s original test for “psychopathy.” (Georgina took this disk when she switched the test results.) Mia reflects on her emotional and psychological tendencies, and she decides to take the test to discover if she has inherited her mother’s psychopathy.