The film begins with a voiceover describing the trench warfare situation of World War I up to 1916. In a château, General Georges Broulard, a member of the French General Staff, asks his subordinate, the ambitious General Mireau, to send his division on a suicide mission to take a well-defended German position called the “Anthill”. Mireau initially refuses, citing the impossibility of success, but when Broulard mentions a potential promotion, Mireau quickly convinces himself the attack will succeed.
Mireau proceeds to walk through the trenches, asking several soldiers, “Ready to kill more Germans?” He throws a private out of the regiment for showing signs of shell shock. Mireau leaves the detailed planning of the attack to Colonel Dax of the 701st regiment, despite Dax’s protests that the only result of the attack will be to weaken the French Army with heavy losses for no benefit.
Prior to the attack, a drunken lieutenant named Roget, leading a night-time scouting mission, sends one of his two men ahead. Overcome by fear while waiting for the man’s return, Roget lobs a grenade and retreats. Corporal Paris, the other soldier on the mission, finds the body of the scout, who has been killed by the grenade, and confronts Roget. Roget denies any wrongdoing and falsifies his report to Colonel Dax.
The next morning, the attack on the Anthill is a failure. Dax leads the first wave of soldiers over the top into no man’s land under heavy fire. None of the men reach the German trenches, and B Company refuses to leave their own trench after sustaining heavy casualties. Mireau, enraged, orders his artillery to open fire on them to force them onto the battlefield. The artillery commander refuses to fire on his own men without written confirmation of the order. Meanwhile, Dax returns to the trenches and tries to rally B Company to join the battle, but as he climbs out of the trench, the body of a dead French soldier knocks him down.
To deflect blame for the attack’s failure, Mireau decides to court martial 100 of the soldiers for cowardice. Broulard persuades him to reduce the number to three, one from each company. Corporal Paris is chosen because his commanding officer, Roget, wishes to keep him from testifying about Roget’s actions in the scouting mission. Private Ferol is picked by his commanding officer because he is a “social undesirable.” The last man, Private Arnaud, is chosen randomly by lot, despite having been cited for bravery twice previously.
Dax, who was a criminal defense lawyer in civilian life, volunteers to defend the men at their court-martial. The trial however, is a farce. There is no formal written indictment, a court stenographer is not present, and the court refuses to admit evidence that would support acquittal. In his closing statement, Dax denounces the proceedings: “Gentlemen of the court, to find these men guilty would be a crime to haunt each of you till the day you die.” Nonetheless, the three men are sentenced to death.
The night before the scheduled execution, Dax confronts Broulard, at a ball, with sworn statements by witnesses attesting to Mireau’s order to shell his own trenches, in an attempt to blackmail the General Staff into sparing the three men. Broulard takes the statements but brusquely dismisses Dax, returning to the ball on the floor below.
The next morning, the three men are taken out to be shot by firing squad. Dax, suspecting Roget for his nomination of Paris, forces Roget to lead the executions. While a sobbing Ferol is blindfolded, Paris refuses Roget’s offer of a blindfold and reacts defiantly to Roget’s meek apology. Arnaud, meanwhile, is so badly injured after having started a fight in prison that he must be carried out in a stretcher and tied to the post. All three men are then executed.
Following the executions, Broulard breakfasts with the gloating Mireau. Broulard reveals he has invited Dax to attend and tells Mireau that he will be investigated for the order to fire on his own men. Mireau storms out, protesting that he has been made a scapegoat, threatening Dax of insubordination. Broulard then blithely offers Mireau’s command to Dax, admiringly assuming that Dax’s attempts to stop the executions were a ploy to gain Mireau’s job. Discovering that Dax was in fact sincere, Broulard rebukes him for his idealism, while the disgusted Dax calls Broulard a “degenerate, sadistic old man.”
The day following the execution, some of Dax’s soldiers are raucously partying at an inn, while Dax observes their behavior from a distance. A captive, attractive German girl is brought onstage for the soldiers’ entertainment, and they clap and whistle lustily at her presence. Their mood shifts, however, as they listen to the girl fearfully singing “The Faithful Hussar” — they hum along with her singing, a strikingly human moment following the unjust execution of their fellow soldiers by firing squad.
They are unaware that orders have come for them to return to the front. Dax allows the men to enjoy a few more minutes before they are called back to duty. His face hardens as he returns to his quarters.