Rookie cop Billy Costigan grew up in crime. That makes him the perfect mole, the man on the inside of the mob run by boss Frank Costello. It's his job to win Costello's trust and help his detective handlers bring Costello down. Meanwhile, SIU officer Colin Sullivan has everyone's trust. No one suspects he's Costello's mole. How these covert lives cross, double-cross and collide is at the ferocious core of the widely acclaimed The Departed.