The novel alternates between two perspectives, those of estranged spouses Simon and Catherine Nicholson, as they recount the 25 years since Simon vanished from their family home near Northampton, England. Their stories unfold through parallel timelines: flashbacks to the past and a tense present-day confrontation in which Simon returns unannounced to the cottage where Catherine still lives.
On a June morning 25 years earlier, Catherine wakes to find Simon's side of the bed empty. She assumes he has gone for a run, but his business partner Steven calls to say Simon missed a client meeting. His wallet remains on the sideboard. By evening, Catherine contacts hospitals and friends, then Roger, Simon's childhood friend and a detective sergeant, who files a missing-person report.
Simon's chapters reveal he does not go for a run. He dresses in the dark, takes a gold Rolex watch his mother Doreen once gave him, and walks to nearby Harpole Woods, where he has been attempting to hang himself for two weeks. On this morning, a voice in his head suggests an alternative: Instead of dying, he can simply disappear and start a new life. He places the rope on the ground and walks away from his family.
Before fleeing England, Simon travels to London to find Doreen, who drifted between his father Arthur and her violent lover Kenneth Jagger, Simon's biological father. A café waitress tells him Doreen died years earlier after Kenneth beat her fatally. Simon visits the dying Kenneth in Wormwood Scrubs prison. Kenneth reveals that Doreen had been packing to leave and find Simon when he attacked her. Simon returns the Rolex and whispers something in Kenneth's ear that the reader does not hear, prompting Kenneth to call his son a monster.
Catherine faces a cascading crisis. Arthur and his wife Shirley accuse her of driving Simon away and tip off police that his body may be buried in her garden; officers dig up the yard and find nothing. Financially devastated, Catherine sells the family car, pawns her wedding rings, and has their furniture removed to stave off repossession. She takes a supermarket job and begins sewing clothes at night, despite her fear that the work will revive memories of their infant son Billy's death. She suffers a miscarriage and swallows painkillers with wine in despair, though she vomits them up before they take effect.
Simon crosses the English Channel hidden in a truck and drifts south through France to Saint-Jean-de-Luz, where he works as a janitor at a backpacker hostel. When a young English traveler named Darren Glasper dies of a heart attack there, Simon steals his passport, recognizing their physical resemblance. He restores the building to its former art deco glory and discovers valuable papers by the architect Pierre Chareau. On New Year's Eve, after the hotel's grand reopening, Simon lights candles in four rooms and walks away as flames consume the building.
Catherine's fortunes slowly turn. Margaret, owner of an upscale boutique called Fabien's, recognizes Catherine's talent for making clothes. When Margaret retires, Catherine buys the boutique using proceeds from selling her share of Simon's architectural business. She has Simon legally declared dead after seven years, holding a funeral to give the family closure.
Simon sells the Chareau papers in Paris and flies to New York as Darren Glasper, plunging into a reckless life of cocaine and constant travel. In Key West, he spots Roger walking with Paula, Catherine's best friend and Roger's girlfriend. She recognizes Simon and confronts him. Terrified she will expose him, Simon shoves her into the path of an oncoming van, killing her instantly. He does not know she is pregnant.
Catherine meets Tom, a divorced solicitor-turned-wood-sculptor, and they begin dating. But after a prison warden calls to report that Kenneth Jagger has died and left a watch for "Simon Nicholson," who visited four days after Simon's disappearance, Catherine checks the wardrobe and finds the Rolex missing. Realizing Simon left voluntarily, her rage poisons the relationship with Tom, and she pushes him away.
Simon's stolen identity unravels when Darren's brother Richard Glasper tracks the impostor through royalty cheques. Simon frames Richard by planting drugs in his luggage and flees to Mexico, where he works at a bordello and falls in love with Luciana, an Italian-born prostitute. After they kill a violent client in self-defense, they begin a relationship. When Luciana's abusive father dies, they move to his villa above Montefalco, Italy, where Simon develops the vineyards into a winery. They have two children, Sofia and Luca. Simon confesses his past to Luciana, who accepts him but insists he must one day face Catherine.
Catherine's children grow into adults. James becomes lead guitarist of a successful rock band called Driver. Robbie develops selective mutism after his father's disappearance, refusing to speak for an extended period, but recovers and studies computer science at Sunderland University. Emily studies fashion and marries Daniel, the son of Catherine's friend Selena. Catherine is diagnosed with a cancerous brain tumor but enters remission after surgery and treatment. Her neurosurgeon, Dr. Edward Lewis, becomes her partner, and they marry in Central Park surrounded by family.
Luciana is diagnosed with incurable cancer. Simon cares for her through the final months and, at her request, administers a lethal dose of morphine. Before dying, she makes Simon promise to tell Catherine the truth. Luca, a fan of Driver, unknowingly corresponds with his half-brother James online. At a concert in Rome, Luca brings Simon backstage, where father and eldest son shake hands without James recognizing him. The encounter compels Simon to honor his promise.
In the present-day confrontation, Catherine notices Simon's mental lapses and realizes he has Alzheimer's disease. He reveals his reason for leaving: On the night of his 30th birthday, he saw Catherine and his best friend Dougie go upstairs together and concluded they were having an affair. He believed their youngest child Billy was Dougie's son, and when Billy slipped underwater in the bathtub, Simon watched without intervening, attempting resuscitation only after hearing Catherine's screams. What Simon does not tell Catherine is that he also murdered Dougie and buried the body in the nearby woods, a fact revealed only to the reader.
Catherine tells Simon what actually happened that night: Dougie, drunk and bitter over his unrequited feelings for Simon, raped her. The footsteps she heard pausing outside the bedroom door belonged to Simon, who walked away without opening it. She never reported the assault because she feared no one would believe her. She tells Simon that Billy could not have been Dougie's child because of the nature of the assault; Billy was Simon's biological son.
Simon's entire justification collapses. Catherine attacks him with a fireplace poker, cracking his wrist, but stops short of killing him. She tells him he does not deserve an easy death and wants his disease to consume him until his only memory is of the son he killed. She throws him out.
In the epilogue, Simon staggers to the churchyard and finds the headstone Catherine erected for him. He accepts that he was the architect of his own misery. He swallows his remaining medication with whiskey, then lies back on the grass beside his empty grave, watches the sky darken, and wonders if God will listen when he apologizes for everything he has done.