A military lawyer comes to question the status quo when he defends three men accused of cowardice.

In 1916, on the Western front during World War I, prolonged trench warfare between the French and the Germans breeds hopelessness among the Allied soldiers. To ensure himself a promotion, ambitious division commander Gen. Mireau accepts the proposition of his commanding officer, Gen. Broulard, that he take the difficult, if not impossible target of Ant Hill, a German stronghold. Broulard's subtle but convincing argument is prompted by his need to silence civilian criticism about the standoff. Mireau then tours the trenches delivering false hope and informing Col. Dax, a former criminal lawyer and commander of three regiments on the front line, that his regiment must take Ant Hill despite knowing that he will lose over half his men. When Mireau boasts that "France is depending on you," Dax replies under his breath that "patriotism is the last refuge of the scoundrel." Mireau then threatens to furlough him from his men, forcing Dax to accept the assignment. That evening Dax sends Lt. Roget, Corp. Paris and Pvt. Lejeune on a reconnaissance patrol to Ant Hill. After crawling yards under wire and over muddy trenches, the three come within several hundred feet of the hill, where Roget sends Lejeune ahead to investigate a noise. When Lejeune fails to return immediately, a panicky Roget throws a grenade at the hill and darts back to the regiment. Paris runs to the hill to save his friend but finds Lejeune has been burnt alive by the grenade. Returning to camp, Paris accuses Roget of murdering one of his own men, but Roget caustically reminds him that no one will believe the word of a corporal over that of a lieutenant. Later that night, after Dax prepares his men to take Ant Hill the next morning, the soldiers discuss the likelihood of surviving, admitting that they fear pain more than actual death. Early the next morning, Dax orders his men out of the trenches and onto the battlefield using his whistle. As Dax presses ahead, his men fall to the left and right of him under heavy enemy fire until he realizes that the second force, Company B, has not taken to the battlefield. Returning to the trenches, Dax finds that his men have fallen back and a cowering Roget has failed to order Company B into action, claiming that the attack is already lost. Meanwhile, Mireau, realizing the battle is lost, orders his troops to open fire on the men in the trenches. When artillery commander Capt. Pelletier refuses to obey, insisting that the order must be in writing, Mireau threatens to arrest him and states that if the troops will not "face German bullets, they'll face French ones." The next day at the headquarters, Mireau accuses Dax of cowardice in the face of the enemy and calls for one hundred of his men to be court-martialed and executed. When Dax snidely offers that they shoot the entire regiment or, better yet, shoot him, Broulard diplomatically suggests that each company commander select one man to stand trial. Powerless to stop his superiors, Dax requests that he be allowed to act as defense counsel for his men. Within hours the three men have been chosen and imprisoned in a cell, where Dax learns why they were chosen: Roget named Paris because he witnessed Roget throwing the grenade at Lejeune, Pvt. Arnaud was chosen by chance and Pvt. Ferol attributes his fate to being a social misfit. At the informal trial held at French headquarters in a lavish chateau, the judges refuse to read the full indictment against the men or have any transcription taken of the trial, despite Dax's protests. The first to testify, Ferol admits that he retreated, but when Dax questions him further, Ferol reveals that he made the decision when he realized he was facing the enemy with only one other soldier alive. When Dax reads Arnaud's citation of merit and bravery for other battles, the court dismisses the evidence as immaterial. While on the stand, Paris admits that he did not leave the trenches because he had been knocked unconscious, but the court insinuates that without witnesses, Paris could be lying. After the prosecutor makes his closing statements accusing the men of creating "a stain on the honor of France," Dax is outraged by the illegal proceedings and warns the court that the crime they commit in finding these men guilty without a proper trial will stain their reputations. The men are returned to their cell where, soon after, a priest announces their guilty verdict and tries to prepare them for their execution by firing squad set for the next morning. When the priest asks Arnaud for his confession, the drunken soldier accuses him of sanctimony and lunges to hit him. Paris stops Arnaud with a punch that throws the soldier into a brick wall, resulting in a near-fatal head injury. Meanwhile, Dax orders the cowardly Roget to be in charge of the firing squad and then goes to Broulard with written testimony from several soldiers regarding Mireau's orders to fire on his own men during the Ant Hill attack, hoping that the general will change the court's verdict. However, the next morning the execution proceeds on schedule. While walking to the firing posts between two long lines of their own comrades, Paris manages to keep his composure and bravery to ensure that his wife and children will have fond memories of him, while Ferol breaks down, sobbing in the priest's arms. Arnaud, who is carried on a stretcher, has his cheeks pinched so that he is conscious as the squad takes aim. Forced to ask the men if they want a blindfold, Roget approaches Paris and apologizes only seconds before all three are shot. Later that night, after Dax is called into Broulard's quarters, the general informs Mireau that Dax has presented him with sworn statements that Mireau ordered firing on his own men. Broulard casually mentions that an inquiry will be necessary to clear Mireau, but all three men realize this will end Mireau's career. After Mireau self-righteously reminds the men that he is a soldier and storms out, Broulard calls Dax his "boy" and offers him Mireau's job. Incensed by his duplicity, Dax calls Broulard a "degenerate, sadistic old man," prompting the general to suggest Dax suffers from sentimentality. Once outside, Dax is drawn by the sound of his men's applause for a German female prisoner who is being forced to sing for them. As the sobbing woman sings a folk song, the soldiers begin to hum along to the familiar melody. Dax must order his men to return to the front immediately, but allows them a few sobering moments as they recognize their humanity in the song as a respite from the war's brutality.