In the early day of World War II, the crew of English cargo ship the SS Glencairn — including Swede Olsen (John Wayne), Englishman Smitty (Ian Hunter), American Yank (Ward Bond), and Irishmen Driscoll (Thomas Mitchell) and Cocky (Barry Fitzgerald) — is sailing from the West Indies to Baltimore. Upon their arrival, their new cargo — a load of dynamite — puts the men ill at ease, as does the chance that there may be a Nazi spy on board and the general loneliness of seafaring.