Billy Costigan (Leonardo DiCaprio)’s Timeline and Summary
- We see Billy go through the police academy, but he gets stopped dead in his tracks when he runs into Captain Queenan and Sergeant Dignam. Despite Billy's great test scores, Queenan and Dignam think he doesn't have what it takes to be a cop. Instead, Queenan offers him a job as an undercover agent because of Billy's family's connection to the criminal underworld of Boston.
- Billy accepts Queenan's deal and goes to jail for assault to help establish his street cred. When he gets out, he moves to the working class neighborhood of Southie to live with his grandmother.
- Billy tries to get on the Irish mob's radar by doing small-time drug deals with his cousin. The mob boss Frank Costello eventually finds out and tells Billy not to do anything more without his permission.
- To up the ante, Billy goes into a convenience store and beats up two mafia guys. He knows that Frank Costello hates the mafia and that Frank will have to get involved if he wants to prevent the mafia from coming after Billy.
- After a violent interrogation, Frank Costello is satisfied that Billy isn't a police rat. So he starts pulling Billy in on jobs and working him into his gang. Meanwhile, Billy finds out that there might be a rat in the police department. This info makes him freak out because he's worried that the rat will expose him to Costello.
- As things get more intense, Billy goes back to Queenan and says he wants to be pulled out of the field. He knows that Costello will find him out and kill him sooner or later, and Billy wants the cops to arrest Costello before that happens.
- While working for Frank, Billy often goes to see a psychologist named Madolyn. He begs her for drugs to help him sleep, and when she finally agrees he asks her out for coffee.
- Billy keeps working for Costello with increasing nervousness.
- Eventually, Billy takes it on himself to find the rat in the police. He follows Costello to one of his meetings with his rat and chases the rat down an alleyway. But he ends up losing the guy without seeing who he is.
- One day, Billy meets with his boss Captain Queenan and gets a phone call from Costello's gang saying that they know their rat is meeting with the cops. Billy realizes that the gang is on its way to kill him, and he barely escapes while the gang murders Queenan.
- With no one else to turn to, Billy visits Madolyn at her apartment and has sex with her. Afterwards though, she says they can't see each other again because she's engaged.
- Back at the gang's hideout, Billy realizes that one of the thugs (Delahunt) has figured out he is the rat. But the guy dies from a gunshot wound before he can tell anyone else.
- As things reach a boil, Billy accompanies Costello to a major drug deal. The cops show up and blow most of the gang away, but Billy escapes.
- Billy shows up at the police station once Costello is dead. He sits down with Sergeant Sullivan to get his money and his identity. But while sitting in the office, he realizes that Sullivan is Costello's rat and runs away. Then he makes two envelopes, one for Madolyn and one for Sergeant Sullivan, and puts them in the mail.
- Billy sets a meeting with Sullivan and ends up trying to arrest him. But he gets shot through the head by another dirty cop before he can get Sullivan back to the police.