A deported gangster trains an Italian convict to take over his operations in the U.S.

Colini, an exiled American gangster living in Sicily, rescues Giordano, a young Sicilian outlaw, from the police. After Giordano is groomed, polished, and renamed "Johnny Cool," Colini sends him on a vengeance mission to the United States to assassinate the men who plotted his downfall and enforced exile. Johnny arrives in New York and quickly kills several of the underworld figures on Colini's list. Meanwhile, he picks up Dare Guiness, a wealthy divorcée who becomes his accomplice, and she is severely beaten by the gangsters as a warning against the vendetta. Soon the FBI becomes involved, and when Johnny and Dare bomb the Hollywood home of gangster Lennart Crandall, the police are able to identify Dare's car. The two separate and plan to meet later, but Dare, realizing that Johnny is a vicious killer, tells his enemies where to find him. She then surrenders herself to the FBI, and Johnny is murdered by the henchmen of one of his victims.