The film's framework plot is provided by the interrogation of former French front-line soldier Albert Maillard by the Moroccan colonial police in November 1920. The central question is: How did he meet Edouard Péricourt?
Following this, Maillard tells the story of the two former comrades-in-arms, which commenced in November 1918. Both Maillard and Péricourt are part of a unit which is led by the egomaniacal and sadistic lieutenant Henri d'Aulnay Pradelle. When an order to cease fire reaches Pradelle, he ignores it and instead in bright daylight sends two soldiers – one of them very young, the other one rather old – on a patrol in the battle field. Soon after that, both of them get shot, seemingly by German troops. Consequently, a battle between both sides break outs. But while the battle takes place, Albert recognizes that the two soldiers have been shot from behind by Pradelle in order to break loose this one last combat. Pradelle now tries to murder Albert, but instead the soldier gets buried alive in a shell crater. After a dead horse's head, in which some breathable air is still encapsulated, momentarily saves his life, it's finally Edouard who digs him out. But just one second later, the rescuer is heavily wounded in the face by an explosion.
In the military hospital Edouard recognizes that, as a result of his heavy injury, his lower jaw is gone. He can't speak anymore, instead he's only able to utter rattling noises, and as a consequence of his severe pain, he develops a morphine addiction. Albert, who doesn't leave his rescuer's side, obtains the opiate for him. He also discovers Edouard's drawings, which deeply impress him. Because Edouard doesn't want to go home to his very wealthy and powerful but also emotionally distant father, who disdains his art, he beseeches Albert to swap his identity against that of a fallen soldier and, in this way, allow him to disappear. The plan is successful and without further ado Edouard Péricourt becomes Eugène Larivière.
In November 1919 Albert and Edouard live together in impoverished conditions in a Paris dwelling. To finance both of them, Albert works in some occasional jobs. He also still has to provide Edouard with morphine, which he steels from other war cripples. Meanwhile, Pradelle is doing very lucrative but also risky business with war graves. A testing commission ascertains first infringements, but Pradelle manages to extort secrecy from the officials. At the same time, Edouard becomes friends with Louise, a war orphan who was given into the custody of their landlady. Reading the newspaper, he also gets wind of the great demand for war memorials in post-war France. Thereupon the gifted artist conceives a plan to design and sell such monuments, while never producing let alone delivering them. With the money, which he aims to make this way, Louise, Albert and himself could then fly from France. Albert, when told of the plan by Edouard and Louise, first refuses to be a part of this criminal undertaking. But it isn't long until even more personal setbacks make him join anyway. Now it's his job to acquire money for the printing of the advertising catalogues.
Meanwhile Edouard’s father mourns his son who he believes dead, whereupon Edouard’s sister Madeleine shows him some wartime drawings of her brother which Albert sent to the family together with the letter of condolence. After that, the Péricourts decide to invite Edouard’s comrade for dinner. During this dinner, Albert finds out that in the meantime Edouard’s beloved sister got married to Pradelle. He also meets the Péricourt’s maid Pauline for the first time and, because of his poverty which is obviously caused by the war, Edouard's father offers him a job as bookkeeper – his original profession – in his banking house. Also, Edouard's father shortly afterwards decides to hold a contest for the best design of a war memorial for all the fallen soldiers of the arrondissement – and thus his son. The winner is awarded the huge sum of 150 000 Francs for the construction of his design. Meanwhile in his new job, Albert begins to embezzle money in order to raise enough money for the production of the catalogues. In the wake of a patriotic frenzy, shortly before July 14, the French national holiday, countless orders come in. The demanded advance payments are transferred to the bank account of the company “Souvenir Patriotique” by the non-existent artist Jules d’Epremont. Albert then gets the money paid out to him, in order to fly to the African colonies on July 14 together with Edouard and Louise.
But when Edouard finds the wedding announcement card of Pradelle and his sister in Albert’s jacket, he instead decides to take revenge on Pradelle first. So he passes a letter to an official of the Ministère des pensions, in which he accuses Pradelle’s company to be responsible for the mistreatment of war victim’s bodies, on which the news have recently been reporting. And indeed, the official Joseph Merlin finds out that Pradelle lets his workers dismember corpses in order to bury them in shorter and therefore cheaper coffins. Pradelle again tries to bribe Merlin into keeping silent about the crime, but Merlin instead reports it to his superiors, using the bribe money as evidence. Now, in case his dark doings become public, Pradelle faces the death penalty.
Meanwhile, Edouard’s father discovers his son’s stylized signature on Jules d’Epremont's submission to the contest and now wants to get to know the artist. But, as a consequence, he must realize that Jules d’Epremont doesn’t exist at all. Thereupon he calls upon his son-in-law, Pradelle, who he actually despises. The lieutenant, who doesn’t just face the death penalty but, after Madeleine found out that he cheated on her repeatedly and after he fulfilled his purpose of impregnating her, was also left by his wife, is assigned the task to find the frauds. He accepts it and in return Monsieur Péricourt guarantees him to use his influence on the minister in charge to gain Pradelle impunity.
In the meantime, Edouard and Louise have moved into the Hôtel Lutetia, where they have huge parties and Edouard’s morphine addiction becomes even worse. Pradelle finds out the whereabouts of the swindler who is still known to him under the name of Jules d’Epremont. He tells it to Edouard’s father who now, as arranged, gains him impunity. Around the same time, the maid Pauline, with whom Albert now has a relationship and who he wants to marry, reads about the swindle with the fake war memorials in the newspaper. When Albert then tells her that he’s involved in this crime, she leaves him. But when he brings her back to the Péricourt’s house, Albert has to watch how Pradelle – who is originally responsible for his and Edouard’s situation – molests her. He then follows Pradelle and, on one of the lieutenant’s construction sites, threatens him with a gun. Pradelle backs away from Albert and tries to appease him by saying that the war was over, but Albert replies that he isn’t so sure about that anymore. Not seeing where he’s going, Pradelle then tumbles backwards into an excavation where lots of sand quickly come rushing down on him. Even though Albert is trying to rescue him, within seconds Pradelle is buried alive – together with a mask in the form of a horse’s head which Edouard made for Albert as a memory of the event which bound them as companions in misfortune and which Albert was carrying with him when confronting Pradelle.
While Albert is now waiting for Edouard and Louise at the Gare de Lyon, so that the three of them could escape together, his comrade is under the strong influence of morphine and lies on the balcony of his suite on the highest floor of the Hôtel Lutetia. Suddenly, his father comes to see him. At first, he doesn’t recognize his son, since Edouard wears one of the many ornate masks, which he made for himself in order to cover his injury. Not even the least bit interested in the mask or the swindle, which cost him 150 000 Francs, Monsieur Péricourt just asks the man before him whether he knew his son – because of the signature on the design for the monument. Soon after that, he recognizes Edouard by his eyes and tells him in the third person that he’s proud to have had him as a son. The two men embrace, then Edouard replies to him “Merci” and takes his own life by jumping of the balcony.
In the framework plot, the interrogation is over when Albert addresses the deceased Edouard by saying the words “Au revoir là-haut” – See you up there. The interrogating officer then sends his subordinates out of the room and allows Albert to escape. As it happens, he’s the father of the young soldier who was shot by Pradelle on 9 November 1918. Outside of the police Building, Louise and Pauline, who obviously changed her mind regarding Albert, wait for him.