Two just-graduated officers from Massachusetts State Police Academy follow opposite sides of the law: William Costigan is assigned to work undercover with the Irish mobster Frank Costello to get evidences to arrest him. His true identity is only known by his superiors Dignam and Oliver Queenan. The protégée of Costello, Colin Sullivan, is promoted in the Massachusetts State Police and is the informer of Costello. Each police officer gives his best effort trying to disclose the identity of the other "rat".