A Sociopath's Guide to a Successful Marriage

Set in the affluent North London suburb of Muswell Hill, the novel is narrated by Lalla Rook, a meticulous, manipulative woman who describes herself as emotionally detached and prone to violent impulses she has learned to suppress. Lalla lives with her husband Stephen, an investment banker, their six-year-old daughter Nelly, and four-year-old son Nathan. The story opens as Lalla prepares for Nathan's birthday party and calmly reveals that she has fatally stabbed an intruder with a vegetable knife after he grabbed her around the neck, severing his axillary artery. Rather than call the police, she proceeds with the party.

Lalla's stresses include Nelly's admissions process for Adams Prep, a prestigious school; her domineering mother-in-law Madeleine's interference; and Stephen's emotional and sexual withdrawal since his father Roger's death. Her master plan is to push Stephen to make partner, move to Hampstead, and have a third child. Her close friends arrive: Sophie, a heavy-drinking teacher in a complicated relationship with her partner Paolo; Aisha, a health-conscious interior designer whose family is considering a move to Abu Dhabi; and Cait, a fearful woman whose husband Owen was exposed as a domestic abuser. During the party, Owen violates his restraining order by sending Cait a threatening photograph. Lalla keeps guests away from the living room with improvised lies.

After the party, Cait returns and sees Lalla wrapping the body through a window. Lalla forces Cait's hand onto the knife handle, contaminating the murder weapon with her fingerprints. Cait refuses to help and flees, her departure captured on the door camera. Lalla moves the corpse to the garage and cleans the scene.

That evening, Stephen confesses he skipped Nathan's birthday. Lalla reflects on her own abusive father, hinting she resolved the situation violently just before turning fourteen. She gets Stephen drunk, has sex with him, then moves the corpse into her Porsche after locating the dead man's blue Toyota Corolla nearby. The next day, she visits her wealthy friend Tor, married to Lawrence, a Conservative politician, whose property has concrete footings under construction for a new pool house. Using Ring doorbell footage, Lalla discovers the intruder had been surveilling her for days. On Sunday, Stephen reports that police are searching for a missing man named Jason Mercer and that an anonymous letter arrived claiming Lalla is not who she says she is. The next day, Lalla and Cait bury Mercer's body in the freshest footing at Tor's property.

Detectives DS Birch and DC Mattoo visit to reveal that Mercer was a suspended police officer facing trial for assault, bribery, and fraud, and that Lalla's name was found in his desk. In Mercer's car, Lalla finds a notebook containing surveillance records of her movements, an alias listed as "MonkeyWarrior," and what she decodes as a meeting at Pancho's café on December 3. She also receives a photocopy of a 1999 newspaper article about Brian Wells, found with stab wounds at home, his wife Margaret Wells arrested, and their 13-year-old daughter taken into care. This exposes Lalla's true identity as Lola Wells.

When Lalla breaks into Cait's empty house to plant the knife, she finds Owen inside, drunk, having soaked the bed in gasoline. He attacks her. In desperation, Lalla strikes a match and drops it on the bed, engulfing Owen in flames, and escapes. Investigators later find Owen's body with a knife wound, and Cait is charged with his manslaughter.

Lalla visits Madeleine at her Kensington mansion, secretly adding foxglove leaves to her tea. Madeleine reveals she traced payments Roger made to someone named "Lola Wells" and connected the newspaper article to Lalla's identity, demanding she leave Stephen or face exposure. Madeleine drinks the poisoned tea and later has a cardiac episode. With Stephen at her bedside, Lalla transfers two hundred thousand pounds from his savings for the Hampstead deposit.

On December 3, Lalla stakes out Pancho's and identifies MonkeyWarrior as Matthew Hollis, her first husband, whom she believed she killed ten years earlier by pushing him off a ledge on Mont Blanc in the Alps. In flashback, Lalla reveals she was homeless when Hollis, an Australian, took her in. They married, but she found his emotional neediness and control intolerable, pushed him off the mountain, and fled to London under a new identity. Hollis survived but was paralyzed from the waist down. His existence makes Lalla a bigamist: She never divorced Hollis, so her marriage to Stephen is legally void, and a lawyer confirms she would be entitled to nothing.

Events accelerate. Lalla pays Aimée, their French nanny, to flirt with Stephen, applies testosterone gel to his skin while he sleeps, and blackmails Josh Krill, a partner at Stephen's bank, threatening to expose his sexual harassment of interns unless he ensures Stephen makes partner. She intercepts Zac Estall, who has been extorting Tor over an affair Tor previously confided. Lalla seduces Zac, extracts a confession, transfers a hundred and fifty thousand pounds from his Bitcoin wallet to her own account, and discovers a video of Lawrence with Zac, gaining further leverage.

In January, Lalla discovers Stephen's four-year affair with Georgette Mallenberg, his first fiancée. Stephen asks for a divorce and reveals he has not made partner. Lalla confirms she is pregnant and retaliates against Krill for betraying their deal by electrocuting him. She blackmails Lawrence and pressures Tor into swapping anonymous candidate numbers on the Adams exam so that the scores of Hero, Tor's daughter, are attributed to Nelly.

Lalla attempts to push Hollis's wheelchair into the Thames but aborts when he reveals his tech company is worth a billion dollars. Hollis proposes, and Lalla accepts, though she discovers he has written a new will excluding her. Later, Hollis confronts her at the same spot, having deduced she deliberately pushed him off Mont Blanc, and holds her at gunpoint. Cait rushes in and shoves his wheelchair into the river. Lalla sends Cait to destroy the will before anyone discovers it. Hollis dies in hospital, and because the will is destroyed, Lalla inherits his billion-dollar fortune as his legal spouse.

Stephen reads Georgie's stolen diary and discovers Madeleine orchestrated the entire affair, buying Georgie a house and engineering their encounters. He ends the relationship. Lalla reveals her bigamy; Stephen, relieved he can walk away without a divorce, apologizes, reveals he made partner, and they agree to a future together. Nelly's Adams result is a rejection: An anonymous tip exposed the candidate swap. Lalla screams not at Hollis's death, which detectives deliver at the same moment, but at the rejection letter.

In a final scheme, Lalla intercepts Sophie's Adams acceptance letter for her daughter Ellie, forges a rejection, and calls Adams posing as Sophie to register the place under Nelly's name. She engineers the removal of Mrs. Pembury, the Adams headmistress, by transferring Bitcoin payments into Pembury's bank account and sending anonymous accusations of financial impropriety to the board of trustees. She posts a compromising image of Tor on Facebook, identifying Tor by name, as retaliation for sabotaging the number swap.

In the epilogue, Lalla reflects at Claridge's. She has inherited Hollis's fortune, bought the Hampstead house, and is expecting a baby girl who may be Zac's child. She and Stephen are remarrying. Cait remains in prison, having pleaded guilty to Owen's manslaughter for the book deal and platform it provides. Georgie was struck by a blue Toyota Corolla, implied to be Mercer's car driven by Lalla, and returned to Dubai. The police concluded Hollis killed Mercer after Lalla planted evidence at his flat and called in an anonymous tip. Lalla is now PTA chair, heir to a billion-dollar AI company, and already eyeing properties in Kensington.