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During World War I, the first American troops to fight in France are veteran Marines who have fought all over the globe.  Company L, returning from the front, is led to its home base of Bar-le-Duc by Captain Flagg and lieutenants Aldrich and Moore, Corporal Kiper, Sergeant Moran and Lipinsky.  Flagg is happy to reunite with his French sweetheart, curvaceous barmaid Charmaine, but is annoyed by her plea to accompany him on his upcoming leave in Paris.  Flagg lies, telling Charmaine that he is married, and the next day, reviews his new troops.

The untrained youths both try Flagg's patience and make him fear for their safety, so he anxiously awaits his new "top sergeant," who will train them for battle.  Unfortunately, the new man is Sergeant Quirt, Flagg's longtime rival and friend, and the two at once resume their ritual of brawling upon seeing each other.  Flagg knocks Quirt down, then rejects his transfer request and orders him to whip the recruits into shape while he is away.  While Flagg is gone, Quirt works the new men hard and romances Charmaine, as another newcomer, young Lewisohn, falls in love with local girl Nicole Bouchard.

Upon Flagg's return, he learns that the company is being sent to the front, and also receives a complaint from Charmaine's father, innkeeper Cognac Pete, that a soldier has trifled with her affections and should be forced to marry her.  Flagg is infuriated to discover that the culprit is Quirt, who had stolen other girl friends from him in the past, but gets his revenge by ordering Quirt to marry Charmaine immediately.  While Quirt is being hustled off to the ceremony, Lewisohn asks Flagg for permission to marry Nicole.  Flagg, who has spent his life avoiding marriage, urges Lewisohn to wait, and then goes to watch Quirt and Charmaine's wedding.  Quirt is nonplussed that Charmaine readily agrees to the marriage, but wiggles his way out of the situation when he deduces the company is about to move out.  Realizing that he cannot jail his top sergeant for disobedience and leave him behind, Flagg grudgingly gives Quirt a reprieve and the men march to the front.

During the prolonged fighting, Flagg is enraged by the deaths of his soldiers, many of whom are barely more than boys.  Needing to capture a German officer who can confirm intelligence reports about enemy movements, Flagg sends Lieutanant Moore on a reconaissance mission.  Moore is killed, however, and the wounded Aldrich goads Flagg and Quirt into going themselves.  As they crawl through the mud, Flagg and Quirt both realize that they love Charmaine and want to give up their long-valued freedom to marry her.

After taking a German-held farmhouse, the Americans succeed in capturing a German colonel, but he is killed in a barrage.  Quirt is injured in the blast, and jovially torments Flagg with the knowledge that he will be returning to Bar-le-Duc first.  When they arrive back at the trenches, Quirt and Flagg are delighted to learn that Lewisohn has captured a German officer, but their joy turns to horror when Lewisohn is killed by an enemy barrage.  Flagg is further infuriated when General Cokely reneges on his promise to return the men to safety after they capture a German.  Flagg bitterly leads his men deeper into enemy territory, while in the village, Quirt sneaks out of the military hospital to visit Charmaine.  Quirt tells Charmaine that he wants to marry her, but before he can, Flagg returns from the front and insists that he will marry the barmaid.

Tired of the senseless killing, Flagg confesses that he is not married and wants to raise a family, but when Charmaine cannot make up her mind between him and Quirt, the two men begin a drunken brawl.  They eventually decide to play poker, with Quirt's pistol as the stake, and the victor is to shoot his rival, then marry Charmaine.  Flagg, who does not hold the winning hand, bluffs and shoots at Quirt as he runs from the inn, but before he can celebrate, Lipinsky announces that the company has received orders to move out again.  The morose Flagg refuses to leave, but then, realizing that his first duty is to his men, bids farewell to Charmaine and urges her to marry Quirt.  As they are marching away, however, Quirt limps after his comrades, and a smiling Flagg hands him a rifle.