Pierre Metard (Jean Pierre Aumont), a French army captain, working with the British Commandos, is given the assignment to find a secret Nazi submarine base in Brittany. He assumes the identity of a dead French soldier, whom he resembles, since he thinks the base is near the soldier's village. He hopes the villagers will help him and hopes none of them will betray him. He finds a sweetheart, Anne Pinot (Susan Peters) and also a charmer, Elise (Signe Hasso), who, unknown to the villagers,is covertly working with the Nazis. After many narrow escapes , he finds the wall behind which the U-boats are hidden. The commandos make a brilliant raid and destroy the submarine base.