2 just-graduated police officers follow opposite paths. Billy Costigan is assigned to work undercover with the Irish mobster Frank Costello to collect evidence against him to enable his arrest. His true identity is known only by 2 of his superiors. On the other hand, the other officer, Colin Sullivan is a protégé of Costello and works as an informer for him. But when it becomes clear to both the police and the mob that there is a mole in their midst, each police officer must give his best to disclose the identity of the other ’rat.’