The movies describes a major theme of “The Departed” as one of the oldest in drama—the concept of identity—and how it "affects one's actions, emotions, self-assurance, and even dreams.” Many years later, an older Sullivan, now in his mid twenties, (Matt Damon) is finishing his training for the Massachusetts State Police with classmates, including fellow cadet Barrigan (James Badge Dale). In another class are Cadet Brown (Anthony Anderson) and Billy Costigan (Leonardo DiCaprio). All four men graduate to become state troopers. Sullivan is a sergeant, and has just passed the state trooper detective test. He goes in to meet with the calm and collected Captain Queenan (Martin Sheen), and the aggressive and cynical Staff Sergeant Dignam (Mark Wahlberg) of the Special Investigations Unit.
When Sullivan exits, Costigan goes in. The undercover division of the Special Investigations Unit wants to assign Costigan, whose family has long had ties with the Boston underworld, to infiltrate Costello's crew. For his service, he gets a "bonus", tax-free payment upon completion of his assignment. To make his assignment believable to everyone, especially Costello, they create a false conviction on Costigan for assault. He will get a four month jail sentence and afterwards probation with mandatory sessions with a psychiatrist. His police academy record and file are concealed from the public, and even the department itself, and the only ones who can access Costigan's file are Queenan and Dignam.
The main characters in this movie have a father-son relationship is a motif throughout the film. Costello (Nicholson) acts as a father figure to both Sullivan (Damon) and Costigan (DiCaprio) and Queenan (Sheen) acts as Costello's foil in the role of father-figure presenting both sides of the Irish-American father archetype.
Social identity is a theory formed by Henri Tajfel and John Turner to understand the psychological basis of intergroup discrimination.