Adjudant-Chef Galoup of the French Foreign Legion reflects on his life from his home Marseille. He recalls his time in Djibouti, where he led a section of men under the command of Commandant Bruno Forestier. Galoup admired and envied many of Forestier's qualities, including his clear affection from the men, and retains a wristband with Forestier's name. Galoup has a Djiboutian girlfriend, and they often go out dancing.

One day, a new recruit named Gilles Sentain joins Galoup's section. Galoup harbors an immediate and seemingly irrational hostility towards Sentain, and vows to destroy him. When Sentain hands a canteen of water to another soldier who is being punished by being forced to dig a large hole in the heat of the day, Galoup chastises Sentain and knocks the water from his hand. Sentain strikes Galoup, who retaliates by taking Sentain into the desert and ordering him to walk back to the base alone. However, Galoup had previously tampered with Sentain's compass, causing him to become lost and collapse from dehydration in the arid salt flats.

Sentain is ultimately found and rescued by locals, but never returns to the base and is presumed to have deserted. His compass is later found by the legionnaires at a local sale of salt-encrusted novelties, and is believed to prove Sentain is dead. On the assumption that Galoup has either killed or attempted to kill Sentain, Galoup is sent back to France by Forestier for a court martial, ending his career in the Foreign Legion. He makes his bed in the immaculate military manner, then lies on top clutching a pistol, and reads aloud the phrase tattooed on his chest: "Sers la bonne cause et meurs" ("Serve the good cause then die"). The film ends with a dream sequence of Galoup at a night club in Djibouti, engaging in a lively acrobatic solo dance to "The Rhythm of the Night". It is heavily implied that he commits suicide.