The film tells the story of the crew aboard a United Kingdom tramp steamer named the SS ''Glencairn'' on the long voyage home from the West Indies to Baltimore and then to England. The crew is a motley, fun-loving, hard-drinking lot. Among them is their consensus leader, a middle-aged Ireland named Driscoll ("Drisk") (Thomas Mitchell), a young Sweden ex-farmer Ole Olsen (John Wayne), a spiteful steward nicknamed Cocky (Barry Fitzgerald), a brooding Lord Jim-like Englishman Smitty (Ian Hunter), and a burly, thoroughly dependable bruiser Davis (Joseph Sawyer), among others. The film opens on a sultry night in a port in the West Indies where the crew have been confined to their ship by order of the captain, yet they yearn as ever for an opportunity to drink and have fun with the ladies. Drisk has arranged to import a boat-load of local ladies, who along with baskets of fruit, have agreed to smuggle bottles of rum on board where, with the acquiescence of the captain, the crew carouse until a minor drunken brawl breaks out and the ladies are ordered off the ship and denied any of their promised compensation. The next day the ship sails to pick up its cargo for its return trip to England. When the crew discovers that the cargo is high explosives, they at first rebel and grumble among themselves that they won't crew the ship if it is carrying such a cargo. But they are easily cowed into submission by the captain and the ship sails, crossing the Atlantic and passing through what they all know is a war zone and potential disaster.