The Departed is set in South Boston where the state police force is waging an all-out war to take down the city's top organized crime ring. The key is to end the reign of powerful mob boss Frank Costello (Jack Nicholson) from the inside.

A young rookie, Billy Costigan (Leonardo DiCaprio) is assigned to infiltrate Costello's mob. While Billy is working to gain Costello's trust, another young cop, Colin Sullivan (Matt Damon) is among a handful of elite officers whose mission is to bring Costello down. But what his superiors don't know is that Colin is working for Costello, keeping the crime boss one step ahead of the police.

Each man becomes deeply consumed by his double life, gathering information about the plans and counter-plans of the operation he has penetrated. But when it becomes clear to both the gangsters and the police that they have a mole in their midst, Billy and Colin find themselves in constant danger of being caught-and each must race to uncover the identity of the other man in time to save himself.