When Leo McMahon, an Internal Revenue agent who closely resembles Blackie Harris, a notorious Chicago gangster, learns that Blackie is coming west to transport a shipment of heroin across the Canadian border, he disguises himself as Blackie in an attempt to intercept the narcotics. Blackie outwits Leo, however, and returns to Chicago with the dope. Blackie is killed, and Leo recovers the heroin with the help of Mazie, a policewoman disguised as a gun moll.