In 1916, during World War I in Northern France, French General Georges Broulard asks his subordinate, General Paul Mireau, to take the Anthill, a well-defended German position. Mireau refuses, citing the impossibility of success. However, when Broulard mentions a potential promotion, Mireau quickly convinces himself that the attack will succeed.

In the trenches, Mireau throws a private out of the regiment for showing signs of shell shock. Mireau leaves the planning of the attack to Colonel Dax, despite Dax's protests that the result will weaken the French Army.

Before the attack, drunken Lieutenant Roget leads a night-time scouting mission, sending one of his two men ahead. Overcome by fear while waiting for the man's return, Roget lobs a grenade, accidentally killing the scout. Corporal Paris, the other soldier on the mission, confronts Roget, who 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 trench after seeing that defeat. Mireau orders his artillery to open fire on them to force them onto the battlefield. The artillery commander refuses to fire without written confirmation of the order.

To deflect blame for the attack's failure, Mireau decides to court-martial 100 of the soldiers for cowardice. Broulard orders Mireau to reduce the number and Mireau arrives at three, one from each company. Corporal Paris is chosen because his commanding officer Roget wishes to keep him from testifying about what happened in the scouting mission. Private Ferol is picked by his commanding officer because he is a "social undesirable". Private Arnaud is chosen at random.

Dax, 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. Nonetheless, the three are sentenced to death and shot by firing squad.

Following the executions, Broulard tells Mireau that he will be investigated for ordering to fire on his own men. Broulard then offers Mireau's command to Dax, assuming that Dax's attempts to stop the executions were a ploy to gain Mireau's job. Discovering that Dax was sincere, Broulard rebukes him for his idealism.

After the execution, some of Dax's soldiers are carousing at an inn. They become more subdued as they listen to and then join in with a captive German girl as she sings a sentimental folk song. Dax leaves without informing the men that they have been ordered to return to the front.