Returning to his old university deeply unsettles Tristan. As Professor Linus Goodacre begins the memorial lecture, Tristan recalls presenting his final-year project, Moralia, 20 years earlier. When his rival, Malcolm Hardy, chuckled dismissively during the presentation, Tristan lost control and assaulted him, ending both his university career and his job prospects. He also remembers discovering his adoption papers at his parents’ home, including a letter revealing his father had impregnated another woman in the year Tristan was born.

In the present, Professor Goodacre discusses longevity factors, concluding that quality human relationships matter most. Gordon’s widow, Elizabeth MacMillan, and daughter, Louisa, sit in the front row. After the lecture, Tristan becomes separated from Vivienne and Melvin in the crowded foyer. He thinks he glimpses Janet at the bar, which unnerves him. His old friend Dave approaches, asking why Tristan stood him up for a planned meeting. After making excuses and walking Dave out, Tristan sees Vivienne and Melvin deep in conversation at the bar. Unable to face her pity about his number, 45, he runs from the building.

At the bar, Vivienne warns Melvin that his number, 61, is next. She questions him about Gordon’s tampered pie and briefly suspects him of being the killer. Melvin laughs and provides an alibi: He was in Barcelona with Christian when Gordon died. Vivienne apologizes, and Melvin confesses he briefly suspected her after Janet’s comment about her missing envelope. Christian calls, asking Melvin to join him for dinner.

Melvin confesses to Vivienne that he now lives with Christian and Mary knows everything. At his 60th birthday party the previous year, Mary and Christian drunkenly argued over whether Melvin preferred mild cheddar or ripe Camembert—both were right, exposing the affair. That night, Mary interrogated Melvin about past infidelities and accused him of planning to leave before her cancer returned. The next morning, she coldly told him to move out. Though Melvin claims Mary is thriving, he privately reflects that even his new life is unhappy and he has been unfaithful to Christian.

Nearby, Gordon’s daughter Louisa argues loudly with Elizabeth, calling her late father a “hypocrite” and an “embarrassment” before storming out. Elizabeth tells Melvin that Gordon was bulimic, which caused their separation. Professor Goodacre arrives and escorts Elizabeth away. Tired of the drama, Melvin leaves, turning off his phone as he walks away.

Eighteen months after Gordon’s memorial, Vivienne awakens from a fugue state at 5:30 in the morning outside a church near her old office. She has lost several hours; a receipt in her bag shows she was at a bar called Unit. At home, she researches fugue states and worries about her worsening physical pain. Cat, now pregnant with her partner Ziggy’s baby, is moving out. In a flashback, Vivienne recalls finally telling Cat the full story: At 18, pregnant with James’s baby, she gave birth to a son who died shortly after. When her mother delivered the news, Vivienne blacked out, experiencing her first fugue state.

Mary calls with devastating news: Melvin is on life support after taking pills at a nightclub on his 61st birthday. At the hospital, Mary explains Christian found him after they took ecstasy tablets. Vivienne suspects the pills contained dangerous PMA. Mary reveals Melvin changed after the dinner at Serendipity’s around three years ago. Vivienne tells Mary about the numbers, the sins, and the deaths. Mary’s phone rings and she rushes away, with Vivienne understanding that Melvin is dying.

Later, examining Melvin’s empty envelope, Vivienne notices an embossed EMB symbol. A search leads her to a printers’ shop near her old office. The next day, at Melvin’s memorial gathering, Christian approaches Mary to express his grief and guilt, but she coldly rejects him. Vivienne and Tristan leave for a restaurant, where Vivienne, distressed by her fugue states and the printers connection, confesses she thinks she is the killer.

Tristan is shocked by Vivienne’s confession. She explains her reasoning: Her fugue states coincide with the deaths, she has no memory during those hours, she found a receipt from near where Melvin was, and she used the printers who made the invitations. Tristan calmly debunks her theory, pointing out she could not have orchestrated Gordon’s or Melvin’s complex murders during brief blackouts.

Reassured, Vivienne wishes Tristan happy 40th birthday and gives him her father’s watch. She asks why he has been distant, and he admits he recently met with Ellie. At the pub, Ellie—pregnant with her second child—talked happily about her life, then told him she forgave him for his behavior when they broke up, devastating him. Shortly after, his Moralia software project was definitively rejected, and he spiraled into deep depression, spending days in bed and canceling plans with friends.

They walk along the embankment toward Hungerford Bridge. When Vivienne links her arm through his, he pulls away. She theorizes the numbers are warnings that can be changed by addressing one’s sin, suggesting Tristan can alter his fate by confronting his inner anger. Tristan snaps that he is ready to die now and runs onto the bridge, climbing over the railing. Vivienne runs after him, pleading desperately. Tristan apologizes, then grabs her and pulls her over the railing with him. They plummet into the Thames together, and Tristan loses consciousness.