The narrative jumps to July 1999 at Thornwood. A week into the project, Will realizes he is falling for Sophie. However, Dominic also flirts with Sophie, making Will jealous. By now, Professor Marlowe has hired an attractive young woman named Eve to help with household tasks at Thornwood and Will sometimes sees Dominic flirting with her. Sophie is preoccupied with the mysteries surrounding Professor Marlowe: She wants to know why Marlowe abandoned his successful academic career at an American university and returned to Britain, and she also wants to know what happened to Barbara (Marlowe’s wife).

Back in the present, the group continues to disagree about how to handle the dangerous situation. Theo insists that Callum’s claim about a secret is likely a bluff to distract them during a robbery, while Will argues they must uncover the secret if they want to survive. Callum comes back to the room and summons Will out into the hallway. Outside, Will notices that Amber and Callum seem like they could be in a romantic relationship.

Will accuses Callum of trapping them in the house; Callum concedes, but states that he didn’t need to lock the back door since it was apparently already locked. Callum shows Will a pile of valuable possessions that he has been looting and gathering from the house. Callum shows Will the dossiers that have been assembled about the six team members and urges Will to reveal the secret if he knows it. Callum promises that whoever is left when the secret is revealed will be allowed to go free: He and Amber are planning to flee with the valuables after the mission is completed. Will is startled when Callum uses the same phrase that Finn used earlier: “everything echoes” (144).

Callum gives Will a diamond ring from the pile of loot and tells him to offer it to Rohan. Callum wants to see if Rohan will accept it; as a cover story for how Will obtained the ring, he is supposed to claim he stole it when Callum wasn’t looking. Will is confused and hesitant but Callum threatens to hurt Sophie unless he obeys.

The narrative returns to July 1999. The team has been working intensively on the dating website, with Professor Marlowe exerting significant pressure on them. One day, Will and Sophie take a rowboat out on the lake; Will considers kissing her or confiding his feelings to her. They are interrupted by commotion on the shore and row back. Lily is showing the other teammates the first set of matches produced by the dating website, based on psychological profiles. The website has been tested with a beta set of profiles, compiled from local volunteers and from the team themselves. Everyone on the team has matched with a stranger, except for Will: His assessment shows that Sophie is his best match. As they discuss the results, Will proposes calling the dating website Butterfly.net (a pun on “butterfly net”).

Dominic interrupts to ask about his match; Lily says the system found no matches, attributing it to a bug. Dominic storms off angrily. Lily reflects on the strange dynamics within Marlowe’s family and explains to the group that Professor Marlowe’s wife, Barbara, was murdered by a serial killer while the couple were living in America.

In the present-day narrative, Will returns to the dining room to join the others. He mentions the stolen goods and Theo interprets this as proof of a burglary. Will insists that they need to solve the mystery of the secret if they want to survive. The group begins to speculate again, and Will wonders if the secret could be connected to Eve, since other than Dominic and Professor Marlowe (both of whom are now dead), she was the only other person at Thornwood. Lily quickly tells him that last she heard, Eve was living and working in London and is unlikely to be connected to these events. Callum and Amber enter the dining room, both armed, announcing time is up.

Will is extremely fearful of what is going to happen next, but Theo continues to bluff and negotiate. Theo is startled when Callum mentions Olivia, demanding to know how Callum could know her name. Since no one volunteers to reveal a secret, Callum explains they will vote to determine who from the group is most likely to know the secret.

The vote begins. Rohan votes for Theo, citing his authority and dismissiveness. Theo votes for Will, accusing him of conspiracy. Georgina also votes for Will. Sophie and Will vote for Theo. Lily hesitates, then chooses Theo. Callum points the shotgun at Theo and counts down, forcing a confession. Theo admits that during the summer of 1999, he blackmailed James (the wealthy financier who was funding the dating website project). James regularly visited Thornwood and Theo uncovered evidence that he was hiring escorts. Theo threatened to make this information public unless James helped Theo advance his career in banking. James complied, and this dynamic allowed Theo to achieve the wealth and status he currently possesses.

Callum says this is not the secret he seeks and prepares to shoot Theo for failing at the task. Theo panics and runs for the door; Callum shoots him in the back as he flees, and Theo dies instantly.

The narrative returns to July 1999. One day, Will, Sophie, and Eve are sitting outside when they hear Georgina screaming from the woods nearby. They hurry to help her and find that Georgina has accidentally stepped on a wasp nest and is now being swarmed by angry wasps. Theo arrives on the scene and rescues Georgina by carrying her into the lake. Will later notices them kissing and realizes that Georgina and Theo have begun a romantic relationship.

After the wasp attack, Will tells Sophie, Lily, and Eve that he saw Professor Marlowe reading about psychopaths. Lily seems unsurprised and tells the group that when she first met Professor Marlowe, his research focused on psychopaths; he has only more recently switched to studying how to use psychology to predict romantic compatibility. Eve shares gossip that Marlowe was forced to leave his previous job in America, but Lily defends the professor, explaining how he convinced her parents to let her continue university.

With Theo dead, the atmosphere becomes even more sinister. Callum orders Rohan and Will to move the body. As they do so, Will seizes the opportunity to offer the diamond ring to Rohan, who refuses to take it and appears disgusted. Will suspects Callum engineered the plan so as to sow distrust between the two former friends. Back in the dining room, Callum gives them another hour to discuss the secret. He warns that if no one confesses, another vote will occur and another person will die.