Young Treasury Agent Elliot Ness arrives in Chicago and is deternimed to take down Al Capone, but he learns that it's not going to be easy, because Capone has the police in his pocket. But Ness meets Jimmy Malone, a veteran patrolman and probably the most honorable one on the force. He asks Malone to help him get Capone, but Malone warns him that if he goes after Capone, he is going to war. They recruit academy cadet George Stone and Treasury Agent Oscar Wallace, who is also an accountant, who wants to prosecute Capone for tax evasion. When they make headway, Capone tries to get them, but they are untouchable.