The Departed

The crime boss Frank Costello persuades a youngster, Colin Sullivan, to join the police force as an inside man. Colin is accepted for training into the State Police. At Police Academy, one of Colin's classmates is Billy Costigan.

When the trainees graduate to the force, Billy is given the task of infiltrating Frank Costello's mob. To gain street-cred, Billy commits an assault, goes to prison, and starts drug-dealing. Pretty soon he is noticed by Costello, who brings Billy aboard his crew.

The criminal and the police both have their inside man.