In March 1944, OSS officer Major John Reisman is ordered by the commander of ADSEC in Britain, Major General Sam Worden, to undertake Project Amnesty: a top-secret mission to train some of the US Army’s worst prisoners and turn them into commandos to be sent on a virtual suicide mission just before D-Day. The target is a château near Rennes in Brittany where dozens of high-ranking German officers will be eliminated in order to disrupt the chain of command of the Wehrmacht in Northern France before the Allied invasion. Reisman is told he can tell the prisoners that those who survive will have their sentences pardoned.
Reisman meets the twelve convicts at a prison under the control of the US Army’s MPs. Five are condemned to death while the others face lengthy sentences which include hard labor. Reisman establishes his authority by stamping on the face of the rebellious Franko who he goaded into attacking him. With a detachment of MPs led by Sergeant Bowren acting as guards, the prisoners gradually learn how to operate together when they are forced to build their own training camp. However, when an act of insubordination is instigated by Franko, all shaving and wash kits are withheld as punishment which leads to their nickname “The Dirty Dozen.” During their training the prisoners are psychoanalyzed by Captain Kinder who warns Reisman that they would all quite likely kill him if given the chance; and rapist/killer Maggott is by far the most dangerous.
With their commando training almost complete, the “Dirty Dozen” are sent for parachute training at a facility commanded by Reisman’s nemesis Colonel Everett Dasher Breed of the 101st Airborne Division. When Reisman’s men run afoul of Breed, especially after Pinkley – under Reisman’s orders – poses as a general to inspect Breed’s best troops, the Airborne colonel attempts to discover Reisman’s mission by having two of his men go beat a confession out of Wladislaw. The convicts blame Reisman for the attack but realize later it was a mistake after Breed and his men arrive at their base looking for answers. Reisman infiltrates his own camp and gets the convicts to disarm Breed’s paratroops, forcing the colonel to leave in humiliation.
However, after Reisman gets prostitutes for the men to celebrate the completion of their training, General Worden and his chief of staff, Brigadier General Denton throw the book at him. Denton, who sides with Breed’s testimony, urges General Worden to terminate Project Amnesty and send them back to prison for execution of their sentences. Reisman defends the prisoners ferociously saying each one of them is worth ten of Breed’s men. Reisman’s friend, Major Max Armbruster, suggests a test: During upcoming war games in southwest England, the “Dirty Dozen” will attempt to capture Colonel Breed’s headquarters. The unit does indeed infiltrate and captures Breed’s headquarters using various unorthodox tactics including deception, theft, and impersonation. General Worden is impressed and green-lights Reisman’s mission.
The men are flown to northern France but one man dies breaking his neck during the parachute drop. With a man down, the mission proceeds with German-speaking Wladislaw and Reisman infiltrating the chateau disguised as German officers. However, all surprise is lost when the psychopathic Maggott begins shooting at anyone (friend or foe) before he is killed. With the sound of gunfire, the Wehrmacht officers and companions retreat to an underground bomb shelter/cellar. After the entrance is barred, they are killed by grenades and gasoline which has been poured down ventilation shafts. In the end, only Reisman, Bowren and Wladislaw escape alive. Back in England, a voiceover from Armbruster confirms that General Worden exonerated the sole surviving member of the Dirty Dozen and communicated to the next of kin of the rest that “they lost their lives in the line of duty”.