A skipper-for-hire's romance with a beautiful drifter is complicated by his growing involvement with the French resistance.

In Fort de France, Martinique, in the summer of 1940, shortly after the fall of France to the Germans, an American named Johnson hires professional fisherman Harry Morgan to take him fishing on Morgan's boat. Johnson complains about the cost of the expedition and the onboard presence of Eddie, a drunk, and Horatio, a native. Refusing to listen to Harry's instructions, Johnson loses a rod and reel belonging to Harry. Fed up with Johnson, Harry cancels the rest of the trip and insists that Johnson pay him for the lost equipment as well as his fees for the past week. Johnson promises to pay what he owes after the banks open the next morning. Back in Fort de France, bartender Gerard, commonly known as Frenchy, asks Harry to rent him his boat for one night to transport some members of the resistance underground, but Harry refuses to become involved in Frenchy's political activities. Later, in the hotel bar, Harry sees attractive young Marie Browning pick Johnson's pocket, and when she leaves the bar, he follows her and demands that she return the wallet. Harry checks the wallet and is surprised to see that it contains enough money in traveler's cheques to pay his fees and that Johnson's plane leaves early the next morning before the banks open. After Marie, whom Harry has dubbed Slim, returns the wallet to the indignant Johnson, Harry insists that he sign some of the cheques, but before Johnson can complete this task, he is killed by gunshots from the street directed at Frenchy's allies. The police detain some of the customers, including Frenchy, Marie and Harry, for questioning. Later that night, Marie tells Harry that she is tired of her footloose life and would like to settle down. In order to earn enough money to put himself back in business and help Marie, Harry agrees to pick up Frenchy's friends. Before he leaves, he buys Marie a ticket on the plane leaving that afternoon for the United States. After picking up Helene and Paul De Bursac, Harry is spotted by a patrol boat, and Paul is wounded before they escape. Harry is surprised to find that Marie stayed in Martinique to be with him. At Frenchy's request, Harry removes the bullet from De Bursac's shoulder and learns that the De Bursacs have been assigned to help a man escape from Devil's Island. De Bursac asks for Harry's assistance, but Harry turns him down. Later, the police, who recognized Harry's boat the previous night, reveal that they have Eddie in custody and will coerce him to tell the truth about the boat's cargo. At gunpoint, Harry forces the police to arrange for Eddie's release and sign harbor passes, so that he can take the De Bursacs to Devil's Island. After Eddie returns, he, Harry and Marie leave Martinique for a more committed life together.