In March 1944, OSS officer Major John Reisman is tasked by Major General Sam Worden, commander of ADSEC in Britain, to lead a covert operation known as "Project Amnesty." This classified mission aims to transform some of the U.S. Army's most notorious convicts into elite commandos, tasked with the assassination of Wehrmacht officers at a château near Rennes. The goal is to disrupt the German chain of command in northern France in anticipation of D-Day. Those who survive the mission will earn a full pardon for their crimes.
Reisman encounters twelve convicts at a military prison run by the Military Police Corps. Among them are the mob-connected thief Franko, former major Wladislaw, black soldier Jefferson, the caring yet hazed Posey, and the dangerously religious Maggott. Five of these men are on death row, while the others face long sentences due to various offenses, including assault and larceny. Though Reisman quickly asserts his leadership, the group initially remains uncooperative. Under the supervision of Sergeant Bowren, the convicts slowly begin to work together when forced to construct their own training camp. However, when Franko's insubordination leads to the withholding of their personal hygiene items, the group earns the moniker "The Dirty Dozen."
As they undergo rigorous training, the convicts are evaluated by Captain Kinder, who cautions Reisman that there's a high probability that at least seven of them might attempt to kill him, with Maggott identified as the most volatile.
With their commando training nearing completion, the Dirty Dozen prepare for parachute training at a facility overseen by Reisman’s rival, Colonel Everett Dasher Breed of the 101st Airborne Division. Interestingly, Breed is kept in the dark about Project Amnesty and tries to uncover details through reconnaissance efforts. Reisman, with the assistance of the convicts, forcefully expels Breed and his men from their camp. Nevertheless, Breed's complaint, along with Reisman treating the Dirty Dozen to prostitutes (excluding Maggott, due to his violent background), raises concerns among the ADSEC officials about continuing the project. In defense of the convicts' training, Reisman agrees to have them compete against Breed's troops in a series of war games to demonstrate their capability. To everyone’s astonishment, the Dirty Dozen successfully capture Breed's headquarters, earning Reisman approval to proceed with the mission.
Once they parachute into northern France, tragedy strikes as Jiminez suffers a fatal injury during the jump, leaving their numbers diminished. The mission advances with Wladislaw and Reisman infiltrating the chateau by masquerading as German officers. Their element of surprise evaporates when Maggott commits a fatal error by killing an officer's wife, forcing Jefferson to eliminate Maggott to save the operation. The ensuing chaos drives the Wehrmacht officers and their guests to retreat into a locked underground bomb shelter. In a bold move, Reisman's team pours gasoline through the ventilation system and Jefferson throws grenades inside, resulting in the demise of the officers and their civilian companions.
After an intense showdown where most of the Dirty Dozen are killed, Reisman, Bowren, and Wladislaw manage to escape back to England. In a poignant end, Worden pardons Wladislaw, providing him a chance to rejoin the military. As the film concludes, Reisman informs the families of the fallen convicts that "they lost their lives in the line of duty."
