My Policeman opens in 1999 and is told in first-person narration from the perspective of Marion, the wife of a policeman named Tom. Marion’s sections of the book are written in direct address to Patrick, Tom’s former lover, whom Marion and Tom have taken in after Patrick suffers a stroke. In this section, Marion moves back and forth between a description of the present and a retelling of the events of her teens and twenties, from the time she met Tom until he asks her to marry him. From the moment she first sees Tom, who is her friend Sylvie’s brother, Marion is captivated by his beauty. They reconnect after he becomes a policeman and she becomes a teacher. Tom teaches Marion to swim, and they increasingly spend time together, often with Patrick. Marion is filled with desire for Tom and is impatient, waiting for him to move their relationship to a level of greater intimacy. Finally, Tom brings Marion to Patrick’s empty apartment when Patrick is out of town and asks her to marry him.  

In Part II, the narrative shifts to Patrick’s journal entries from 1957, in which he also recounts meeting Tom. Patrick, who works at an art museum, meets Tom when Tom responds to a traffic accident. Like Marion, Patrick is struck by Tom’s beauty and becomes driven to develop a relationship with him, despite his wariness of the police. Patrick invites Tom to model for a portrait project, which leads Tom to visit him at the museum. Patrick takes pleasure in introducing Tom to the art there, and Tom finds that Patrick, like Marion, helps him develop intellectually. After a portrait-sitting at Patrick’s apartment, the two have sex, after which Tom disappears from Patrick’s life for three weeks. When Tom returns, Patrick determines that in order to remain in Tom’s life, he will have to reconcile himself to the reality that Tom has a girlfriend and, ultimately, a wife. At the end of the section, Tom asks Patrick if he can share him with Marion, and Patrick says yes. 

In Part III, the narration returns to Marion’s voice, picking up the story’s chronology where Patrick’s journal left off, interspersed with a description of their lives in 1999. In the 1999 narration, Tom has been distant from Marion for many years, swimming every morning and spending the rest of the day out of the house. Patrick has had at least one stroke and is unable to care for himself. Marion and Tom decide to take Patrick in, which is Marion’s idea, though she and Tom have not seen him in decades. In 1958, after Tom and Marion marry, the three are still close, and Marion remembers when Patrick let Tom and Marion use his vacation cottage for their honeymoon, joining them mid-week. Upon their return, Marion’s coworker and friend Julia had asked Marion directly if Patrick was gay. The suggestion had shocked Marion. Now, Patrick is taking Tom on a trip to Venice, which, in light of the suspicion that Patrick is gay, enrages Marion. After Julia tells Marion that she, too, is gay, Marion sends an anonymous letter to Houghton, Patrick’s boss at the museum, telling him that Patrick is gay and raising concerns about his interactions with children. 

Part IV of the book consists of Patrick’s journal entries about his life in prison. Patrick was accused by police of acts of gross indecency with a man he had met at the Argyle, a spot frequented by gay men. These allegations are false, but, hoping to shield Tom from guilt by association with him, Patrick confesses. Marion speaks in Patrick’s defense at his trial, only to have the prosecutor read entries from Patrick’s journal that implicate Tom. In prison, Patrick continually thinks of Tom and the time they spent together in Venice, where he felt free to love openly. When Marion visits him, he asks her to tell Tom to come see him, but she says that he won’t. In prison, Patrick is able to get ahold of a new journal thanks to the protection of Bert, a powerful prisoner who likes to hear educated men like Patrick talk. After Marion’s visit, instead of telling Bert his usual stories of Shakespeare or history, Patrick tells him the tragedy of a policeman and the older man who loves him. Disgusted by Patrick’s openness about his sexuality, Bert and his friends brutally beat Patrick. 

The final section of the book returns to Marion’s voice in 1999. Determined to read her manuscript to Patrick before he dies, she works hard to convince Tom to talk to him. She picks up her story in 1958, when Tom returns from Venice. She imagines Patrick will be fired after Houghton reads her letter, but life continues normally for several weeks. She apologizes to Julia for having a negative reaction to Julia’s disclosure, but Julia has already made plans to move to a different school. After Tom neglects to come home one night, Marion rushes to Patrick’s apartment, assuming she will find Tom there and intending to confront them both, only to find that Patrick has been arrested. Marion confesses to Sylvie that she wrote the letter to Houghton, and Sylvie advises her to keep it a secret from Tom. When Tom returns home, Marion assures him she will never leave him. Tom grieves Patrick’s imprisonment, resigns his job at the police department before he can be investigated, and becomes a security guard. In 1999, Marion convinces Tom to read her confession to Patrick. While he reads, she packs a suitcase and leaves, intending to begin her new life by finding Julia.