FBI ace Buddy, identical to slain mafia boss Barton, goes undercover in the Loguet family to gather arms-smuggling evidence. Using Barton's crescent birthmark, he survives power grabs by young master Luca and tests from Barton's stepmother Suzanne. He falls for Barton's wife Elise, wins over mixed-blood bodyguard Haruko, and outmaneuvers Suzanne to seize the San Francisco docks.