Set in 1944 during World War II, the film follows the titular penal military unit of twelve convicts as they are trained as commandos by the Allies for a suicide mission ahead of the Normandy landings.

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 turn some of the U.S. Army's worst convicts into highly skilled commandos to eliminate Wehrmacht officers at a château near Rennes, disrupting the German chain of command in northern France ahead of D-Day.

These five are condemned to death for murder, while the others (Giplin, Pinkley, Sawyer, Lever, Bravos, Vladek, and Jiminez) face lengthy sentences, including hard labor for crimes such as assault, larceny, mugging and impersonation.

Overseen by MPs led by Sergeant Bowren, the convicts gradually learn to operate together when they are forced to build their own training camp.

Later, Breed's testimony combined with Reisman rewarding the Dirty Dozen (with the exception of Maggott, as his crime is the rape and murder of one) with prostitutes at the end of their training, prompts the ADSEC staff to consider terminating the project and sending the men back to serve their original sentences.

Reisman defends the convicts' training and agrees to have them compete against Breed's men in war games to test their mettle.

Breed finds this ridiculously humorous, but to his surprise, the Dirty Dozen successfully capture his headquarters and Worden allows Reisman to resume his mission.

Upon parachuting into northern France, the German-speaking Wladislaw and Reisman infiltrate the chateau disguised as German officers to help the rest of the team inside.

However, Maggott breaks cover by murdering an officer's spouse and firing at his teammates before he is killed by Jefferson for compromising the mission.

After a firefight with the chateau guards and arriving reinforcements that kills nearly all of the Dozen, Reisman, Bowren and Wladislaw escape back to England alive.

A repeated rhyme was written into the script where the twelve actors verbally recite the details of the attack in a rhyming chant to help them remember their roles while approaching the mission target: The cast included many American World War II veterans including Lee Marvin, Robert Webber and Robert Ryan (U.S. Marine Corps); Telly Savalas and George Kennedy (U.S. Army); Charles Bronson (U.S. Army Air Forces); Ernest Borgnine (U.S. Navy); and Richard Jaeckel and Clint Walker (U.S.

The credit scenes at the American military prison – alluded in the movie to be Shepton Mallett – were shot in a courtyard at Ashridge House in Hertfordshire.

In Arch Whitehouse's article in True Magazine, he claimed that all the enlisted men were full-blood Indians, but, in reality, only their leader Jake McNeice was one-quarter Choctaw.

The parts of the Filthy Thirteen story that carried over into Nathanson's book were not bathing until the jump into Normandy, their disrespect for military authority, and the pre-invasion party.

Barbara Maloney, the daughter of John Agnew, a private in the Filthy Thirteen, told the American Valor Quarterly that her father felt that 30 percent of the film's content was historically correct, including a scene where officers are captured.

Unlike the Dirty Dozen, the Filthy Thirteen were not convicts; however, they were men prone to drinking and fighting and often spent time in the stockade.

The critical consensus reads, "Amoral on the surface and exuding testosterone, The Dirty Dozen utilizes combat and its staggering cast of likable scoundrels to deliver raucous entertainment.

[32]In another contemporaneous review, Bosley Crowther called it "an astonishingly wanton war film" and a "studied indulgence of sadism that is morbid and disgusting beyond words"; he also noted: It is not simply that this violent picture of an American military venture is based on a fictional supposition that is silly and irresponsible. ...

But to have this bunch of felons a totally incorrigible lot, some of them psychopathic, and to try to make us believe that they would be committed by any American general to carry out an exceedingly important raid that a regular commando group could do with equal efficiency—and certainly with greater dependability—is downright preposterous.

John Cassavetes is wormy and noxious as a psychopath condemned to death, and Telly Savalas is swinish and maniacal as a religious fanatic and sex degenerate.

Charles Bronson as an alienated murderer, Richard Jaeckel as a hard-boiled military policeman, and Jim Brown as a white-hating Negro stand out in the animalistic group.

[24]Art Murphy of Variety was more positive, calling it "an exciting World War II pre-D-Day drama" with an "excellent cast" and a "very good screenplay" with "a ring of authenticity to it".

Aldrich sets up dispensable characters with no past and no future, as Marvin reprieves a bunch of death row prisoners, forges them into a tough fighting unit, and leads them on a suicide mission into Nazi France.

Apart from the values of team spirit, cudgeled by Marvin into his dropout group, Aldrich appears to be against everything: anti-military, anti-Establishment, anti-women, anti-religion, anti-culture, anti-life.

Overriding such nihilism is the super-crudity of Aldrich's energy and his humour, sufficiently cynical to suggest that the whole thing is a game anyway, a spectacle that demands an audience.

In The Dirty Dozen: The Deadly Mission (1987), Telly Savalas, who had played the role of the psychotic Maggot in the original film, assumed the different role of Major Wright, an officer who leads a group of military convicts to extract a group of German scientists who are being forced to make a deadly nerve gas.