During World War II, the crew of the tramp steamer Glencairn enjoys a final night of carousing in the West Indies before shipping out to pick up a load of dynamite in Baltimore. Among them are the young, good-natured Ole Olsen (John Wayne), saving his pay to buy a small farm in his native Sweden; paternal Irishman Driscoll (Thomas Mitchell); and tortured English aristocrat Smitty (Ian Hunter). After stowing the explosives in Baltimore, the crew is on edge, fearful that rough seas might detonate their volatile cargo. They're also concerned about the increasingly isolated Smitty, who they believe may be signaling the Nazis.