The novel opens with an anonymous interior monologue expressing fear that buried secrets might be uncovered, establishing the central tension of concealment and discovery.
In November 1905, Jeremy "Jem" Kite, a clerk at the Registrar-General's Bureau in Somerset House, London, receives an anonymous typewritten letter: "Jeremy Kite is a murderer. He killed Toby Feynsham. Ask him why." Jem explains that he attended Oxford with Toby and belonged to a close circle the press dubbed the "Seven Wonders" of St Anselm's College. Toby was murdered a decade earlier, and the case was never solved. When Jem's supervisor shares the letter with the office, Jem arrives the next morning to find "Murderer" scrawled on his desk. He resigns in fury and channels his anger into purpose: finding who sent the letter and who killed Toby.
Jem retrieves a photograph of the seven friends in costume from their college production of Shakespeare's
Cymbeline. The group comprised Toby Feynsham, the charismatic leader; Nicholas "Nicky" Rook, a sardonic English student; Hugo Morley-Adams, a wealthy history student; Aaron Oyede, a Black medical student and the first of his race admitted to St Anselm's; Ella Feynsham, Toby's twin sister and a chemistry student at the affiliated women's hall; Prudence "Prue" Lenster, a sharp-minded mathematics student; and Jem himself, a scholarship boy with a club foot and the group's only commoner. Using public records, Jem tracks their current lives: Hugo is a Liberal MP, Ella works in chemistry, Aaron is a doctor, Prue married shortly after the murder, and Nicky remains at St Anselm's as a lecturer.
Flashbacks to their Oxford years establish the group's dynamics. In 1892, Toby befriended Jem on his first day and assembled the circle, declaring he and Nicky would "collect the interesting people." In Toby's rooms, Aaron cut his finger on a gilt-encrusted Sicilian stiletto used as a paperknife, the weapon that would later kill Toby. A flashback to 1893 shows Nicky taking Jem to the Old Quad library rooftop, where their conversation vibrates with unspoken longing. By early 1895, Toby loses his place in the line of succession and takes his frustration out on Nicky, leaving bruises on his wrist. That night, Jem and Nicky kiss for the first time, beginning a brief, intense affair.
Jem's investigation reveals that identical letters were sent to Hugo's fiancée and to Aaron. A critical piece of evidence links all the suspects: Toby's door was jammed after the murder using a trick only the seven knew, yet none told the police. Hugo doubts Aaron and Ella's alibi, suspecting Ella fabricated it to protect Aaron from a racist detective. When Jem visits Prue, now widowed and grieving her son Joseph, who died from a fall and drowning, she names Nicky as the killer without hesitation and accuses the group of treating her and Jem as pets. Jem notices a discrepancy: Prue's son was born six months after her wedding, meaning he was likely conceived around the time of the murder.
A flashback to the group's catastrophic final evening at the Mitre Hotel reveals the breaking point. Toby publicly exposes Nicky and Jem's sexual relationship, sneering at Jem as "Tiny Tim." Nicky, desperate to deflect Toby's fury, dismisses their affair as a "trivial pastime," devastating Jem. Toby attacks Ella and Aaron's planned marriage with racist slurs and threatens Aaron. Hugo proposes to Ella as a rescue; she refuses and storms out. None of them told the police about this evening, and their silence helped the killer escape.
Jem travels to St Anselm's to confront Nicky. Over several days, they share meals and revisit the library rooftop, where Nicky apologizes for dismissing Jem at the Mitre, calling it the decision he most wishes he could change. They sleep together, though Jem tells Nicky he still cannot rule him out. From the college Master, Dr. Earnshaw, Jem learns that Toby visited him before his death to ask about procedures for accusing a fellow student of a serious offence. Jem sends provocative letters to the other four, claiming he knows the secret behind Toby's intended accusation. Aaron arrives furious, and Jem bluffs that he already knows Aaron's secret. Aaron confesses: He assisted in performing safe but illegal abortions on women who would otherwise have turned to dangerous alternatives. Toby discovered this and blackmailed Ella into ending her engagement to Aaron. Jem concludes that Aaron suspects Ella killed Toby and has been protecting her with the alibi for a decade, which is why they never married.
Events escalate when Jem's room is ransacked, his notebook stolen, and oil poured on the staircase steps, causing him to fall. A page from Nicky's published translation is left on his desk to implicate Nicky. That evening, Jem issues an ultimatum, and Nicky confesses. After the Mitre evening, he confronted Toby, who demanded Nicky inform on Aaron or face exposure for his relationship with Jem. When Nicky refused, Toby picked up the stiletto and attacked him. In the struggle, Nicky stabbed Toby, who died almost instantly. Nicky jammed the door using the trick they all knew and carried on in silence for a decade.
Leaving Nicky's room in anguish, Jem is attacked by a figure in the fog. Aaron arrives and the attacker flees. Jem tells Aaron and Ella that the killer confessed and leads them to Nicky's room. A crucial revelation emerges: Aaron and Ella have each spent a decade suspecting the other of the murder. Ella is furious that Aaron doubted her alibi, insisting that doubt, not Nicky's act, destroyed their relationship. Ella quotes
Cymbeline: "Live, and deal with others better." Aaron argues that Nicky's execution would serve no purpose and asks Nicky and Jem to be alive and well for breakfast.
The next morning, the four identify Jem's attacker as Hugo by his running gait. Hugo has Jem's stolen notebook containing Prue's address and has driven toward her village. They race to Aldbury and find Hugo inside Prue's cottage, where Prue is pale and terrified. When Hugo claims any encounter was consensual, Prue cuts him off: "I said no and you would not stop." Jem accuses Hugo of killing Toby, a deliberate bluff, and Hugo's denials effectively admit to the attacks on Jem and his assault on Prue. Ella negotiates terms: Hugo will pay reparations to Prue and Jem and never contact them again, or face exposure. Hugo, facing the ruin of his political career and ducal marriage, agrees.
The five remaining friends sit in Prue's parlour around a fire, sharing toasted cheese and tea. Prue confesses she wrote the anonymous letters out of grief after her son's death. Nicky responds that they owe her gratitude for spurring Jem to act. Ella tells Prue the killer will not be named, that the matter is over, and apologizes for being a poor friend. Aaron and Ella exchange cautious glances, saying only, "We'll see." Nicky offers to help Jem rebuild his life, and Jem accepts. Jem raises a toast: "To absent friends. Forgiveness. What happens next."