A cavalry unit, having crossed the Sahara to relieve a besieged French Foreign Legion fort, arrives to an eerie silence — the enemy has vanished, and the post's walls and ramparts are defended by dead men. The fort's commander, slain by a bayonet through the heart, clutches a letter that links the riddle of the desert massacre to another mystery, the long-ago and far-away theft of a sapphire known as the Blue Water. It was the scandalous disappearance of the Blue Water that led to the self-exile of Beau, the oldest of the Geste brothers.