The film was adapted from a novel by Helen MacInnes, which was serialized in The Saturday Evening Post from May 2 to June 20, 1942, as Cross Channel, and became a bestseller when published as Assignment in Brittany.

Free French Captain Pierre Matard is assigned the task of locating U-boat pens concealed somewhere on the coast of occupied France.

He is chosen due to his striking resemblance to Bertrand Corlay, a suspected Nazi collaborator held by the British and a native of the area in which the pens are believed to be.

From Elise, who consorts with and aids the Germans, he cleverly learns the pen is located in Saint Lunaire, a town on the coast.

Though a number of civilians, including a child, refuse to betray Matard (and are subsequently shot), Plehec's young nephew Etienne reluctantly identifies him.

After they depart, Madame Corlay hears German soldiers approach, so she grabs the pistol and allows herself to be captured to shield the others.

Aboard, Matard finds Anne and Etienne, who had found out the Germans have learned about their involvement and are forced to flee their homeland.