While the story goes off in half a dozen different directions (if you smell fish in the theater it’s because the film’s got its fair share of red herrings), there’s really only one plotline that matters: the race to uncover moles hidden inside the mob and the Massachusetts State Police. Leonardi DiCaprio stars as Billy Costigan, a rookie cop recruited straight out of the academy to infiltrate Boston crime boss Frank Costello’s world.
Costigan worms his way into Costello’s good graces – mostly by kicking the crap out of anyone who glances his way. But Costello’s got an ace up his sleeve in the form of Colin (Matt Damon), an overly eager, overachieving cop who feeds the gangster insider info on potential busts. Colin was practically raised by Costello and even calls him ‘dad’ when he phones in tips.
It’s not long before Costigan figures out the police department’s been compromised. Sullivan soon comes to the same conclusion about Costello’s gang. With their lives literally on the line, the two play a deadly game of cat and mouse in which they try and force the other into revealing himself without giving away their own identity.