In 1906 North Africa, a French Foreign Legion regiment marches across the desert. Some years earlier, at Geste Manor in England, Sir Hector Geste awaits the birth of a son whom he plans to name Beau. However, Sir Hector’s wife dies giving birth to a daughter. Later, Sir Hector visits the Wormwood & Call Boys’ Orphanage in search of an heir he can raise to be a military hero. When he spots Obediah Spittle, a blond-haired, blue-eyed boy, punching the other children, Sir Hector finds his Beau. Miss Wormwood, the proprietor, insists Sir Hector also adopt Obediah’s “identical” twin. Sir Hector renames the boys Beau and Digby Geste. Years pass and the boys are raised alongside their sister, Isabel, becoming an inseparable trio. While Beau is handsome and debonair, Digby is less attractive and physically awkward. Sir Hector grooms Beau for greatness, but Digby is there mainly to follow orders. One day, Sir Hector announces that England is at war and his regiment has been called to duty. He shows Beau the “Blue Water” sapphire, the foundation of the Geste fortune, and asks him to guard the jewel until he returns. Years later, Beau, Digby, and Isabel reach adulthood and Sir Hector returns triumphantly from the Sudan with a much younger bride, Lady Flavia Geste. That night, Sir Hector has a heart attack while making love and lies on his deathbed. Flavia wastes no time in making plans to sell Geste Manor and the Blue Water, and sending Isabel and Digby to a nunnery. She has other plans for Beau. However, the Blue Water disappears during a brief blackout and Beau convinces Flavia to wait until morning before calling the police. Early the next day, Beau disappears and Digby and Isabel discover a note from their brother admitting that he stole the Blue Water for safekeeping. He warns them not to look for him, especially in North Africa. Digby destroys the note, confesses to the crime, and is sentenced to 956 years of hard labor. Meanwhile, somewhere in Morocco, Sgt. Markov and Corp. Boldini greet Beau and a new group of recruits. Beau writes to Digby updating him on his experiences and Digby vows to escape from prison and join him. Later, Flavia visits Digby and tells him that she knows Beau has the Blue Water. Digby refuses to reveal his brother’s whereabouts, but Flavia has sex with the prison Governor so that he will covertly help her stepson escape. Back in Morocco, the new legionnaires learn they will be departing the following day to defend Fort Zindeneuf, where they will likely die. Meanwhile, Digby escapes, makes his way to North Africa, reports for duty and reunite with Beau. Flavia follows Digby, seduces the Legion’s commandant, General Pecheur, and bribes Sgt. Markov to retrieve the Blue Water from Beau. The regiment departs the next morning for the march to Fort Zindeneuf. In the desert, they are attacked by a Sheikh’s Arab army and lose their flag, but Beau and Digby recapture the flag to rally the spirits of their comrades. The next day, the march to the fort continues and Digby experiences a mirage. As he later rejoins the march, General Pecheur and Flavia visit the Sheikh. Pecheur is financing the Sheikh’s army to profit from munitions sales and is not happy that the war is losing money; he orders the Sheikh to attack Fort Zindeneuf and leave no survivors. Later, Markov and his men reach the fort, and to boost morale, the Sgt. throws a regimental dinner. When Flavia and Beau share a dance, she informs him that she plans to return to England with both him and the Blue Water sapphire. Flavia and Beau make love in the desert and she tells him she does not want him to be at the fort when the Arabs attack. Meanwhile, Pecheur and his second in command, Captain Merdmanger, anticipate the carnage and sneak away from the fort before dawn. Beau returns to warn the legionnaires of the impending assault, causing them to desert en masse. As Markov fights Beau for the Blue Water, Beau gives him a fake sapphire that he had hidden in Markhov’s artificial leg. When Markov and Boldini depart thinking themselves rich men, Beau confides to Digby and Flavia that the real sapphire is in a safe deposit box in Paris, France. At that exact moment, in England, Sir Hector finally dies. Back at the fort, Beau stages his own death by burning his uniform and leaving his identification tags in the ashes. He and Flavia flee after he knocks Digby unconscious, leaving his brother behind to be the heroic last survivor. Pecheur and Merdmanger return to discover Beau’s “remains” and give him his desired burial at sea by flushing the ashes down the toilet. Digby refuses to divulge what happened, but when facing a firing squad, he elects to tell the truth. Three months later, Digby returns to Geste Manor and is reunited with Isabel. Meanwhile, Beau and Flavia relax on a beach and enjoy their fortune.