Set in the late 19th century, the story follows three orphaned brothers, Beau, John, and Digby, who are raised by their wealthy aunt. When a valuable family heirloom, the Blue Water sapphire, mysteriously disappears, Beau abruptly decides to leave and join the French Foreign Legion, followed by his brothers who are also intent on protecting him and uncovering the true thief. The harsh reality of their new lives sets in quickly as they are stationed in North Africa and subjected to brutal training under the sadistic Sergeant Markoff. As they endure physical and emotional trials, their bond and loyalty to one another are tested.

While stationed at an isolated desert fort, tensions escalate as the garrison faces constant threats from Tuareg tribesmen. Amidst the siege and treachery within their own ranks, the brothers' resolve is pushed to its limits. The atmosphere grows increasingly desperate and fraught with danger as they navigate betrayal, camaraderie, and survival against overwhelming odds. Through flashbacks and unfolding intrigues, the film builds toward a climactic confrontation that encapsulates themes of honor, sacrifice, and brotherhood.