Humphrey Bogart plays Harry Morgan, owner-operator of charter boat in wartime Martinique. Morgan's right-hand man is Eddie (Walter Brennan), a garrulous alky whose pet question to anyone and everyone is "Ever get stung by a dead bee?" While in port, Harry is approached by Free French activist Gerard (Marcel Dalio), who wants to charter Harry's boat to smuggle in an important underground leader. Adopting his usual I-stick-my-neck-out-for-no-one stance, Morgan refuses. Later on, he starts up a dalliance with Marie Browning (screen newcomer Lauren Bacall), an attractive pickpocket. In order to help Marie return to America, Harry agrees to Gerard's smuggling terms. He uses his boat to bring resistance fighter De Bursac (Walter Molnar) and De Bursac's wife Helene (Dolores Moran) into Martinique. The Vichy police, suspecting that something's amiss, hold Morgan's pal Eddie hostage, tormenting the poor rummy by denying him liquor. Predictably, Morgan comes to Eddie's rescue and manages to escape Martinique, with the delectable Marie as cozy company.
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.