Major de Beaujolais leads a relief column to Fort Zinderneuf - and encounters a baffling and macabre series of mysteries. So begins P. C. Wren's famous story of brotherly love, self-sacrifice and death-or-glory adventure in the French Foreign Legion.

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. Now Beau has disappeared.

John has fled to the Foreign Legion in search of his missing brothers. At the Legion recruiting office in Paris he hears news of them and encounters the sinisterly 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 Sgt-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.