Will arrives at an imposing Georgian townhouse in Notting Hill (a wealthy neighborhood in London). The townhouse is the home of Theo and Georgina, who married after meeting while working on the dating platform project together. The couple have invited all of the original team members to a dinner party celebrating the late Professor Sebastian Marlowe (who has recently passed away). Will is both excited and anxious about seeing his former colleagues after 25 years; since he is a writer struggling to find a subject for his next project, he wonders if he will find inspiration at the dinner. Will encounters Rohan, who is also arriving for the dinner party, and as they approach the house, Will hears a faint, distressed cry. Georgina and Theo greet their guests and usher them inside.

Theo engages a high-tech security system that locks the front door with multiple bolts. He explains that the system is necessary due to burglaries and concerns about squatters in the neighboring empty houses. Will observes the lavish and beautiful home, including a portrait depicting Theo and Georgina’s two daughters: Mia and Olivia. Mia (the younger) is spending the night at a friend’s house and Georgina is evasive about Olivia. When Sophie arrives, Will is startled to see that she is accompanied by a man.

Will has been looking forward to seeing Sophie because he has unresolved feelings for her, dating back to the summer of 1999, and he is newly single. He is relieved to learn that the man who enters with Sophie is not her husband: Theo introduces the unknown man as Finn, Professor Marlowe’s former assistant. Will and Sophie talk about their current lives: Sophie makes jewelry and lives off an inheritance in her grandmother’s cottage; Will teaches creative writing but has never published a novel. As the conversation unfolds, Will feels that something is “off” about Finn but wonders if he could merely be awkward in the presence of a group of old friends.

The group tours the house; it once belonged to crime novelist Rupert Chadwick, who allegedly built a secret passage which the family has never located. Downstairs in the kitchen, they meet Callum, the chef, and his red-haired assistant, Amber, whose appearance briefly reminds Will of Eve (the young woman who worked at Thornwood as household staff). Callum and Amber have been hired for the evening.

The narrative flashes back to 1999. At this time, Will was 22, a recent university graduate, and unsure about his plans for the future. He responded to a cryptic newspaper advertisement, offering opportunities to work for an Internet start-up. At the interview, Will first meets with Theo (who has already been hired on as the project manager) and completes a series of psychometric tests, including the Belbin Team Inventory. Next, Will meets Professor Sebastian Marlowe and Lily, who explain their vision: creating a dating website using psychological testing to generate meaningful matches, unlike the superficial sites currently available. Marlowe describes how he met his wife, Barbara, through a 1960s computer dating program at Harvard. He offers Will the role of wordsmith, responsible for all the site’s text. The job requires living on-site at Sebastian’s home, Thornwood, for the summer, with generous pay and stock options. Swept up by Sebastian’s enthusiasm and vision, Will accepts.

The narrative returns to the dinner party. Lily arrives, apologizing for being late due to a conflict with her ex-wife over childcare. The six team members from the dating platform project are now all present. They toast Sebastian’s memory. Finn asks why they haven’t seen each other in 25 years and it is clear that tensions exist among the former team members and about the end of the project (the dating platform was never launched). Unexpectedly, Theo and Georgina bring up the subject of their older daughter, Olivia, and explain that she is missing.

Theo and Georgina reveal that they have not seen Olivia for 11 months. Olivia decided to take a gap year before beginning university and initially planned to spend this time travelling. However, Olivia began a relationship with a boy named Felix, who discouraged her from traveling. After discovering Felix was seeing other girls, she became withdrawn. Almost one year earlier, Georgina came home to find that Olivia was not in the house, and she has not been seen since. Her phone’s last signal came from the house, and her journal is missing. The police believe she is a runaway, possibly being harbored by an unknown friend, since her passport has not been used.

In light of this information, the mood of the party becomes more somber. When Theo tries to let Rohan out so that the latter can smoke, he is unable to find the key to the back door. Due to the complex security system, this is the only door that can readily be opened. Rohan and Will begin to feel nervous about the reality that they will not be able to exit the house until the key is found.

The narrative returns to 1999; Will arrives at a small train station near Thornwood (which is an isolated estate in the English countryside). He meets Sophie, who has also just arrived, and Professor Marlowe’s nephew, Dominic, picks them up and drives them to Thornwood. Dominic explains the team will live and work in the main house while Sebastian lives in a separate cottage. The first hints of rivalry between Will and Dominic emerge when Dominic looks appraisingly at Sophie and tells Will that she is out of his league.

Back at the dinner, Georgina cannot find the back door key either. Meanwhile, Lily discovers no one has a cell phone signal. Georgina dismisses it as a common issue due to the house’s thick walls. As food begins to be served, Will and Theo talk together; Will shares his observation that Finn seems to know suspiciously little about Marlowe’s research yet claims to have worked closely with him for years. Will also observes Finn moving around, apparently eavesdropping, and exchanging a knowing look with Amber. When Finn catches Will watching, he forces a fake smile.

Will pulls Sophie into Georgina’s study to share his suspicions about Finn. He lists evidence: eavesdropping, writing in a notebook, the meaningful look with Amber, and making errors about information he would have known had he actually worked alongside Marlowe. Will speculates Finn could be a con man, burglar, or psychopath. Sophie dismisses Will’s theory as paranoia. She then confronts Will about his hot-and-cold behavior on their last night at Thornwood (in 1999), asking why he was so angry with her. Will hesitates, missing the opportunity to explain. Sophie says they should just enjoy dinner and leaves.

Will finds Lily still struggling to get a cellphone signal. She reports that the Wi-Fi is down. Lily also mentions that she heard a strange cry when she arrived at the house (the same sound that Will heard upon his arrival). Lily, Will, and Rohan try the front door and discover the security system is malfunctioning and they cannot get out. Theo and Lily examine the security hub but cannot fix it; the system appears to be in lockdown mode. Theo finds a landline so Lily can call her children. Will feels increasingly frightened, suspecting someone has deliberately trapped them inside.

The group sits down to begin eating. Lily returns after successfully calling her children on the landline and tells Will the security system is in lockdown mode, though she cannot determine if this happened randomly or deliberately. Finn questions Lily about her current work project. When she gives only a vague answer, Finn asks if it relates to her work with Professor Marlowe and makes a cryptic comment that “everything echoes” (94).

Finn asks Will about his writing. Will admits he is working on a novel based on their summer at Thornwood. The group seems horrified. The conversation turns to Dominic; days after the team members all left Thornwood, they received word from Professor Marlowe that his nephew had tragically drowned in the lake on the estate. None of them attended his funeral.

When Finn excuses himself to use the bathroom, Will follows him. After searching the house, he cannot find Finn anywhere and concludes that Finn must have gone into the kitchen. Will encounters Sophie just outside the kitchen door; by this point, she is also becoming suspicious of Finn. Will and Sophie begin to hear sounds of a struggle inside the kitchen, and fear that Finn is attacking Callum. Abruptly, the kitchen door opens to reveal Callum and Amber holding shotguns: Finn has been beaten and tied up.

Callum shoves Finn into a cupboard under the stairs and then he and Amber force Will and Sophie upstairs at gunpoint. They rejoin the group in the dining room, who are now all held captive. Callum demands everyone’s mobile phones, which Amber collects. Theo assumes the motive is robbery and offers money, but Callum dismisses him. Callum announces he and Amber are not there to rob them but to discover a secret from the summer of 1999. They explain that at least one of the six original team members knows this secret and “we’ll know it when we hear it” (110). Callum says he will leave them for one hour to discuss the secret among themselves, with Amber guarding the door. He warns them not to try escaping and implies that anyone who does not cooperate will be killed.