Though certain elements of the movie adaptation may differ slightly from the source material, the film will largely follow the plot of Roberts's book.
The "My Policeman" book tells the heart-wrenching story of a closeted policeman named Tom (Styles) and his whirlwind love triangle with Marion (Corrin) and Patrick (Dawson). Set in 1950s Brighton, Tom and Marion's seemingly perfect love story turns on its head when Patrick, a museum curator, enters Tom's life. The two men strike up an affair, but they conceal their love from the world, as homosexuality is still illegal at this point. To avoid any speculation, Tom and Marion tie the knot, while Marion and Patrick agree on an arrangement to share Tom. However, things become increasingly complicated as the throuple reach a breaking point.
The story bounces back and forth between the '50s and the '90s, at which point Patrick reenters the married couple's lives, leading them to recall their tumultuous love affair.