France, 1916. At the height of World War I, General Broulard (Adolphe Menjou), commander of the French armies on the Western Front, persuades the slightly mad General Mireau (George Macready), who's ambitious for a promotion, to launch a suicidal attack on an impregnable German position called the 'Ant Hill' located near Verdun. The unit to make the attack is the 701st Regiment, commanded by Colonel Dax (Kirk Douglas) who will personally lead the attack.
On a personal inspection of the trenches by Mireau, many of the men of the 701st are exhausted by earlier battles and their morale is moderate at best. Dax tries to persuade Mireau to not to order the attack for the men of the 701st are not ready, but Mireau insists that the attack be carried out.
On the day of the attack, under the cover of an artillery barrage and German counter-battery fire, Dax and his men go over the top and advance, many of them with difficulty, over the broken ground towards the Ant Hill. Some of the men refuse to leave the trenches. The crazed and desperate Mireau orders Captain Rousseau (John Stein), the artillery commander, to fire on the French positions, but Rousseau refuses. Meanwhile, the attack is a failure and Dax and his men fall back to their own positions.
Afterward, Mireau convinces Broulard that discipline demands a sacrifice: one man will be selected from the three companies to be court-martialed and Colonel Dax, a former lawyer, volunteers to defend them. The men are Privates Ferol (Timothy Carey), Arnoud (Joseph Turkel), and Corporal Paris (Ralph Meeker). Paris is chosen by the corrupt Lt. Roget (Wayne Morris) who is trying to prevent Paris from exposing him of murdering a fellow soldier during an earlier nighttime skirmish.
But the whole military trial is a farce: under the sinister and watchfull eye of Mireau, the prosecutor, Major Saint-Auban (Richard Anderson), tries to have the three men condemned to death as an example for the other soldiers in the regiment for cowardice and mutiny, while Dax throws up an impressive defense of the men's conditions and gets Paris to testify about Rogets' cowardice and of being drunk and disorderly during the battle.
But in the end, the panel judges (clearly intimidated by Mireau sitting nearby) don't buy any of it and quickly sentence the three men to death. That evening, Ferol, Arnound, and Paris in their cell talk about the insanity of war and their situation.
Dax tries to save his men by crashing a formal dinner party and informing Broulard of the order given to Captain Rousseau by Mireau to fire upon the troops during the battle. But Broulard takes no account of it and Ferol, Arnoud and Paris are shot at dawn the next day. Dax forces the cowardly Lt. Roget to be the firing squad captain as a means of further intimidating him.
Later during a meeting in a lavish chateau with Dax, Mireau, and Broulard, the clearly arrogant and psychotic Mireau shows no remose for the executions and is pleased with the results of the trial. But then Broulard reveals a plan: to have an inquiry opened on Mireau about the order he gave to Rousseau to fire on their own men during the attack. Mireau, in an unconvincing tone, denies ever making such an order, and leaves. Subsequently, Broulard offers to give Dax the promotion he earlier offered to Mireau. But Dax, disillusioned with the corrupt and incompetence of the entire general staff, refuses the promotion and rejoins his men of the 701st who are at a local bar listening to a German cafe-singer (Susanne Christian) perform.
