The SS Glencairn is a British cargo ship in port in the West Indies in the spring of 1940. The men complain about lack of shore leave, but they soon find out that they will have to carry a cargo of ammunition from Baltimore to London, right through the German war zone. Ole (Wayne) is a Swedish sailor who wants nothing more than to take his wages, go home to Sweden, and see his mother again. Smitty (Hunter) is an Englishman who has some kind of dark secret. Driscoll (Mitchell) is an old salt who is determined that Ole will in fact go home, rather than blowing his money on drink once they reach port. But before Ole can go home, they have to make a highly dangerous voyage through waters being patrolled by the German Kriegsmarine and Luftwaffe.
- The Alcoholic: When they're underway, Smitty needs alcohol so bad that he ransacks the captain's quarters for booze. Later it comes out that he was an officer in the Royal Navy before he was drummed out due to his drinking.
- Blowing Smoke Rings: Yank does this while the Glencairn is tied up at port at the beginning of the movie. Later, he does this again when someone gives him a smoke while he lays in his bunk, dying from a punctured lung.
- Burial at Sea: Done for Yank after he dies of a punctured lung suffered while trying to retrieve the anchor during a storm.
- Downer Ending: The SS Amindra, the ship that Driscoll got shanghaied onto, is revealed at the end to have been torpedoed, with all hands lost at sea.
- The Drunken Sailor: The men go on a binge once they make port in London. They know that this is a bad idea, that it usually ends up with them blowing all their money and having to sign up for another voyage. They do it anyway.
- Ensemble Cast: There really isn't a single lead, as the film focuses on the various sailors of the Glencairn. Wayne, the biggest star in the cast, has little to do until late in the film.
- Fanservice Extra: The very first shot of the movie is of an island girl who seems to be trying to wriggle out of her dress.
- Father Neptune: Driscoll the old salt, who's something of a father figure and/or leader to the other sailors.
- Gentlemen Rankers: Smitty, The Alcoholic with a Dark and Troubled Past. It turns out that Smitty used to be an officer in the Royal Navy before he was kicked out due to his drinking.
- Ooh, Me Accent's Slipping: Let it be said that John Wayne's Swedish accent isn't completely terrible. Although one wonders if that's why Ole has so little dialogue.
- Press-Ganged: After the men of the Glencairn make port in London, they go out to celebrate. The Amindra needs another sailor however, so the men of the Glencairn are led to a scuzzy bar and deliberately plied with drink, so that Ole can be separated from them and forcibly brought aboard the Amindra. When the gang figures this out the charge aboard the Amindra and rescue Ole, but Driscoll gets knocked out and winds up getting stuck aboard.
- Slice of Life: Really not a lot of conflict, in fact, hardly a plot as such. Instead, an examination of the lives of the sailors of a cargo ship, in port and at sea.
- Slipping a Mickey: Ole is drugged in a seedy bar, in order to press-gang him onto the SS Amindra.
- Witch Hunt: The sailors of the Glencairn, paranoid and afraid while trying to make their way through German-patrolled Atlantic waters, wind up convincing themselves that Smitty is a German spy. They grab him and are on the verge of lynching him when they find out that he's actually a former Royal Navy officer who was cashiered for drinking.