In November of 1942, the French submarine, the "Casabianca,", escapes from German-held Toulon, and, upon joining the Free French forces at Algiers, is sent on a secret mission to Corsica to take two secret-service agents to make contact with the underground there. The agents contact the Marquis resistance forces, and learn they are ready to revolt but lack the needed arms and ammunition. The submarine is sent back to Corscia with the necessary weapons for the resistance-fighters, and also returns with trained troops to assist the resistance forces in attacking the Germans.