Three adopted English brothers join the French Foreign Legion in North Africa, after one of them steals their adoptive family's famous heirloom sapphire.
Beau, John, and Digby Geste are three inseparable, adventurous brothers who haven been adopted into the wealthy household of Lady Brandon. When money in the uppercrust household grows tight, Lady Brandon is forced to sell her most treasured jewel the mighty "Blue Water" sapphire. The household gets it out for one final look, the lights go out and it vanishes stolen by one of the brothers, no doubt. That night, Beau, Digby, and John each "confess" and slip out, John leaving behind Isabel, whom he loves. They all join the Foreign Legion, and Beau and Digby are split from John and put under the command of the ruthless and sadistic Sergeant Markoff. Things begin to get hairy as the rest of the Legionaires plot a mutiny against Markoff, in the midst of an attack by Arab hordes.