Years into the future, we see Colin Sullivan (played by Matt Damon) going through police academy and working toward being a cop.
Meanwhile, we see another guy named Billy Costigan (played by Leonardo DiCaprio) taking the final police exam.
Next thing you know, Colin and Billy are both graduating. Frank Costello drives by to say hi to Colin and to give him a present. It looks like Colin is now a mole cop working for the mob.
Next, we see Colin in the office of Special Investigations, which is led by Captain Queenan and Sergeant Dignam. Queenan seems like a nice enough guy, but Dignam is a real jerk.
Right after Colin leaves, Billy Costigan gets up and heads into the office. We find out in his interview that his family connections are all in the rough neighborhood of Southie, which just happens to be where Frank Costello does all his mobbing.
Billy also has an uncle who was killed for being involved in organized crime. Another uncle named Tommy was busted for selling guns. Sergeant Dignam is more than skeptical about Billy's background.
Meanwhile, we see Colin get promoted to staff sergeant immediately, and he sits in on a special meeting for the police department's best and brightest. This is a special unit working to mess with organized crime in Boston, in cooperation with the FBI.
The leader of the meeting is Captain Ellerby, who shows us a picture of Billy Costigan's uncle Jackie after he was shot and killed. He also shows some pics of other organized crime big shots and ends with a picture of Frank Costello.
Back in Special Investigations, Queenan and Dignam tell Billy that he'll never be a State Trooper. They think his SAT marks are way too high and that he's just not cut out for the job.
Meanwhile, we see Billy at his mother's bedside as she dies of cancer. Along the way, he cuts ties with the upper-class part of his family.