One Tough Cop is inspired by the true story of Bo Dietl, the most-decorated police officer in New York City history. The film revolves around Dietl's investigation of the savage rape and mutilation of a Harlem nun, the "most heinous crime in the history of New York City," according to then-mayor Ed Koch. The crime inflames New York citizens and media, prompting police officials to assemble a task force, but removing Dietl and partner Duke Finnerty from the case. They continue their own unauthorized investigation, using street contacts that include Dietl's childhood friend Richie LaCassa, now a mob lieutenant. As Dietl's and Finnerty's detective work surpasses that of the task force, their relationship is tested by Finnerty's excesses, and Dietl's affair with LaCassa's mistress, Josephine "Joey" O'Hara (Gina Gershon). Meanwhile, an FBI probe into Dietl's "mob ties" endangers his reputation.