Training
Wright choses 13 convicts, from whom he intends to select his final Deadly Dozen: Joe Stern, Eric "Swede" Wallan, Ernesto "Pops" Ferucci, brothers Ronald and David Webber, Maurice Fontenac, Martinez, Francis Kelly, Ballews, Sturdivant, Chacon, Spencer and Hallet (who dies during training). This time around the dozen only have one week of training, and with the help of Sergeant Holt, Major Wright begins whipping his men into shape.
Meanwhile, in France, Colonel Krieger captures French Resistance leader Paul Verlaine and his British contact, who had parachuted in with news of the forthcoming mission. After Krieger has the two men executed, Pierre Claudel, a scientist at the monastery loyal to Vichy and the Nazis, identifies Verlaine as the man who has been seen at the monastery conversing with lead scientist, George Flamands. A suspicious Krieger orders increased security at the monastery.
Back in England, General Worden appears at the training site and reveals the intelligence leak and increased security to Major Wright, and informs him that the dozen will now enter France by sea instead of air, as all flights into that section of France were being heavily monitored.
As their training comes to an end, Major Wright throws a party for his men, giving them one last night of debauchery before embarking on their suicide mission.
Mission
Upon coming ashore, the dirty dozen march to their rendezvous point in the French forest, where they meet their contact, Marie Verlaine, the daughter of Paul Verlaine, and her fellow resistance fighters. Marie informs Major Wright that the scientists' families have been brought to the monastery, thus complicating the rescue portion of the mission.
To avoid Krieger's patrols and checkpoints, Major Wright decides that they should travel by river, and the dozen attacks a German patrol boat, successfully taking control of the boat with only one casualty, Sturdivant. From here, the dozen travel down the river, eventually making their way to a Resistance farmhouse, where the men are allowed to rest, while Major Wright and Fontenac go to the monastery dressed as monks to meet with Flamands and inform him of their plans.
One night Fontenac attempts to rape Marie, until he is caught by Stern and Wallan, Wallan pins him to the wall and tells him "You move and I will cut your throat out." Stern however suggests to leave it with Major Wright, but Wallan wants to finish off Fontenac himself, Marie soon finds out that the Dozen are really criminals, Marie asks Wright why they are murders and psychopathic criminals in the US army, as Wright tells the story, Marie states that after the mission she will kill Fontenac herself.
Later that night, Wright and Fontenac re-enter the monastery to trigger the attack from the inside, while the others infiltrate the monastery compound. Wright and Fontenac are followed by the SS sergeant from the gate after the sergeant became suspicious when Fontenac called him by his rank. After being confronted, the major shoots the sergeant and the gunfire starts the attack proper.
While the dozen battle the German troops in the monastery's courtyard, Major Wright destroys the communications center, Stern makes his way into the basement and Fontenac opens the back door allowing Marie, Swede, Martinez and the Resistance men in with the explosives however Fontenac attempts to escape and flee to Spain but Swede prevents this from happening and tells him he will shoot him if he tries to escape, Fontenac follows orders in the end, Wright then heads back out into the courtyard, where, he, Sergeant Holt, the Webber brothers, Spencer and Kelly secure the area, while Ballews and Chacon give covering fire from the balcony, as Marie and Martinez set the incendiary fuses, accidentally igniting the chemicals, which eventually causes an explosion.
As the multiple groups meet up, Ferucci is wounded in the shoulder while Chacon and Spencer are killed in the courtyard. The group then proceeds to the scientists' living quarters, where they find not only the scientists and their wives, but also Fredric Flamands, George Flamands son, and also their family's children as well. The group is also notified that Flamands and his wife Julia have been taken to a Wagner recital at a villa as guests of a German general. Major Wright decides to take everyone with them, and that he and Stern must head to the villa to rescue the Flamands. The scientists and their families are boarded into a truck and head to the spot where a British plane is to pick everyone up. Major Wright sends this truck on a back road while sending two decoy trucks driven by Martinez, Ballews and the Webber brothers on the main roads.
Major Wright and Stern, posing as German officers, enter the villa and remove Flamands and his wife despite being confronted by Pierre Claudel (who was quickly subdued by George Flamands), and in a German staff car head to the rendezvous point.
German patrols stop the two decoy trucks and destroy them, both the Webber Brothers, Martinez and Ballews are all killed. Realizing they have been tricked, Krieger and his men race to the area where the plane has landed and begin firing upon it, as the scientists and their families are being loaded. Fredric panics and calls out for his mother and father, however Fontenac managed to carry him to safety but a mortar shell hits Fontenac and knocks Fredric to the ground and also wounding Fontenac, also Wallan manages to save Fredric in time, and Kelly as well, Fontenac tells Major Wright that it "doesn't look like I'll make it home," he sacrifices himself by manning a machine gun and drawing the fire of the approaching patrols, giving the plane enough time to be boarded and take-off despite coming under heavy fire. The weary soldiers, and the scientists and their families all breathe heavy sighs of relief as the plane embarks on its trip back to England.
Besides Major Wright and Sergeant Holt, the surviving members of the 'dirty dozen' include Joe Stern, Eric 'Swede' Wallan, Ernesto 'Pops' Ferucci and Francis Kelly, who earn their freedom.
