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 protegee 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".
