A husband-and-wife detective team look into the murder of one of her friends, whose father--a prominent scientist--has been kidnapped. They find themselves up against a sinister crime organization headed by a mysterious figure known only as "Z".
The scene of action embraces Tangier, Toulon and the Mediterranean, where Pierre Fresnay, Rolf Wanka and Kim Peacock, cast as captains of French, German and English boats, respectively, unite to rescue the passengers of a neutral boat that is about to be engulfed in a cloud of poison smoke released by a smuggling ship.