The inciting incident is the disappearance of a precious jewel known as the "Blue Water". Suspicion falls on the band of young people, and Beau leaves England to join the French Foreign Legion in Algeria, followed by his brothers, Digby (his twin) and John. After recruit training in Sidi Bel Abbes and some active service skirmishing with tribesmen in the south, Beau and John are posted to the small garrison of the fictional desert outpost of Fort Zinderneuf, while Digby and his American friends Hank and Buddy are sent to Tanout-Azzal to train with a mule mounted company. The commander at Fort Zinderneuf (after the death of two more senior officers) is the sadistic Sergeant Major Lejaune, who drives his abused subordinates to the verge of mutiny. An attack by Tuaregs prevents mass desertion (only the Geste brothers and a few loyalists are against the scheme). Throughout the book, Beau's behaviour is true to France and the Legion, and he dies at his post. Digby, Hank and Buddy arrive with a relief column from Tokotu that reaches Fort Zinderneuf too late. Together with John (the last man standing at Zinderneuf) they desert and experience a long trek during which Digby is killed in a skirmish with Arabs.