Harry Morgan and his alcoholic sidekick, Eddie, are based on the island of Martinique and crew a boat available for hire. However, since the second world war is happening around them business is not what it could be and after a customer who owes them a large sum fails to pay they are forced against their better judgement to violate their preferred neutrality and to take a job for the resistance transporting a fugitive on the run from the Nazis to Martinique. Through all this runs the stormy relationship between Morgan and Marie "Slim" Browning, a resistance sympathizer and the sassy singer in the club where Morgan spends most of his days.