In March 1942, at the Royal Marines base in Portsmouth, England, Capt. Hugh Thompson, a career officer, meets his new superior, the unconventional Major Stringer. Stringer, an experienced yachtsman, has been promoted to the rank of major because of his proposal to lead a special unit of canoes and divers up the Gironde River to the Bordeaux harbor in order to blow up the German blockade runners docked there. When forty-three Marines arrive at the base to volunteer for the mission, Stringer insists on interviewing each one individually. After eliminating his first cut, Stringer decides to test the others by dropping them off 300 miles from the base, dressed in German uniforms and carrying no money. The group is then instructed to return to the base as quickly as possible. By bicycling, riding the rails, jogging and other ingenious solutions, eight men make the grade: Clarke, Ruddock, Stevens, Lomas, Booth, Cooney, Bradley and Todd. Following Thompson's orders, Sgt. Craig begins to drill the men, and when Stringer objects, Thompson terms them ill-disciplined rabble and expresses his contempt for the major's lax methods. After the unit fails miserably during a training mission, Stringer turns to Thompson for advice and Thompson archly observes that the men must conform to military regulations. Under Thompson's guidance, the men undergo a strict training routine while Stringer teaches them how to handle canoes and live explosives. When permission comes to mount the mission, Stringer and Thompson get drunk to celebrate. Inebriated, Thompson confides that he has been passed over for promotion because his company commander blamed him for the failure of his first mission. While Thompson bitterly rehashes his career, the rest of the unit becomes involved in a drunken bar brawl. When Ruddock fails to return to the base with the others, the now sober Thompson goes to Ruddock's house and there finds Ruddock's wife ensconced with a lover. After locating the troubled Ruddock in a pub, Thompson knowingly drives him back to the house, where Ruddock throttles his wife's paramour. Once Thompson and Ruddock return to the base, Operation Cockleshell commences as the canoes are loaded onto a submarine bound for the Gironde River. Once underway, Stringer warns that each two-man canoe team is responsible for their own salvation, and that the success of the mission is more important than the lives of the men. As they near the Gironde, a German artillery ship shoots a depth charge at the submarine. When the force of the explosion knocks Lomas to the floor, unconscious, Thompson insists on taking Lomas' place in Ruddock's canoe. Soon after, the canoes are launched in turbulent waters, and when a crushing wave overturns one of the crafts, the men are left to fend for themselves in the choppy seas and soon are captured by the Germans. The other six paddle to land where they hide in the tall grass, waiting for night to fall. After the tide recedes, they climb back into their canoes and head for Bordeaux. When Craig's canoe is spotted by the Germans, he capsizes the vessel and blows it up with a grenade, after which he and his companion are taken prisoner. As the two remaining canoes, manned by Thompson, Ruddock, Stringer and Clarke, continue upriver, the Germans mount an intensive search for them. Upon reaching the enemy ships, the Marines dive into the water and attach deadly mines to their hulls. Alerted to their presence, the Germans converge upon the harbor and capture Thompson and Ruddock. After the Germans ram Stringer's boat, some friendly French fisherman pull Stringer and Clarke out of the water, saving them from imminent danger. Later that morning, as the mines detonate, blowing up the German ships, Thompson and the five other Cockleshell heroes are executed before a German firing squad for refusing to divulge their mission. Disguised in overcoats, Clarke and Stringer, the only survivors, take to the French roads to escape back to England, satisfied that their mission was a success.
