Somewhere in England, war correspondent Manning arrives at the home base of a Free French air squadron led by Captain Freycinet and is particularly impressed by Jean Matrac, a gunner.  Later, he asks Freycinet for Matrac's story: At the outbreak of the war, Freycinet receives orders to return to France from Southeast Asia.  Also on board the ship, the Ville de Nancy , is Major Duval, a follower of Marshal Philippe Pétain, and some of his men.

They soon receive word that the Germans have broken through the Maginot Line.  Shortly after passing through the Panama Canal, the crew spots a suspicious boat containing five nearly dead men.  The men—Matrac, Petit, Renault, Marius and Garou—claim to be Venezuelan miners trying to return to France, but Duval suspects that they have actually escaped from the penal colony at Devil's Island.  Captain Patain Malo refuses to lock up the men as Duval demands and they are allowed to work for their passage.

After Freycinet warns the men of Duval's suspicions, Renault admits that they are fugitives from Devil's Island and explains how they escaped.  Petit was imprisoned for killing a policeman while defending his farm.  Garou murdered his sweetheart during a lover's quarrel.  Marius is a safecracker; and Renault is a deserter from the Army.  On Devil's Island, the horrible conditions drive them to plan an escape with the help of Grandpère, an older convict who served his term but is not allowed to leave the island.

As their leader, the men choose Matrac, who was sentenced to Devil's Island for his political activities in France: In 1938, Matrac is a journalist and fervent anti-Nazi.  His newspaper is destroyed after he denounces Édouard Daladier for signing the Munich Pact with Adolf Hitler.  Matrac and Paula, his girlfriend, escape to the countryside where they are married, but soon discover that Matrac is accused of murdering a pressmen who was killed during the attack on the newspaper.  Matrac is convicted of murder and sent to Devil's Island.

As the men prepare to leave, Grandpère, a patriotic Frenchman, insists that each one swear to fight for France should they succeed in their escape attempt.  When Freycinet hears their story, he agrees to help them.  After Pétain signs an armistice with Germany, Malo fears that his cargo--valuable nickel ore--will fall into German hands if he docks in Marseille, so he changes course for England.  When Duval discovers the change in plans, he and the Pétain loyalists try to take over the ship, but are thwarted by the other sailors and the convicts.  One of Duval's men manages to radio their position to the Germans, however, and several men are killed in an aerial attack before Matrac shoots down the German plane.

On his arrival in England, Matrac learns that he has a son whom he has never seen.  Whenever possible on a mission, he flies his plane over Paula's house in France and he drops a letter to her.  Tonight, however, Matrac's plane is badly damaged and he dies holding a letter to his son, which Freycinet later reads at his graveside.