Major de Beaujolais leads a relief column to Fort Zinderneuf and encounters a baffling and macabre series of mysteries.

Lady Brandon has had a mysterious Indian visitor while Beau is hiding in a suit of armour, and John in a chest. When the visitor leaves, John is taken from his airless hiding-place, apparently lifeless.

The Blue Water has been stolen, and no one will confess to the theft. Beau has disappeared.

John has fled to the Foreign Legion in search of his missing brothers. He hears news of them at the Legion recruiting office in Paris and encounters the sinister friendly Boldini.

Reunited in the Legion, the Geste brothers have made two good friends, the Americans Hank and Buddy, and one dangerous enemy: the sadistic and tyrannical Sergent-Major Lejaune.

Sergeant-Major Lejaune now believes Beau is carrying a valuable jewel, and is determined to have it. A first attempt at theft is a failure. Then Lejaune announces that the squad is to be posted to Fort Zinderneuf where the Geste brothers will be in his power

The mutiny at Fort Zinderneuf has been averted by an Arab attack. Hopelessly outnumbered, the Legionnaires fight and die bravely while Sergent-Major Lejaune makes his final attempt to steal Beau's jewel.