A column of the French Foreign Legion arrives at the remote Fort Zinderneuf, assigned to relieve the garrison. Upon their arrival, they discover the fort scarred by Tuareg raids, and American Beau Graves, Guy Stockwell, is the only survivor. His badly injured arm is amputated, and when asked what happened, his story unfurls in a tense flashback.
Beau's column had been serving under Lieutenant De Ruse, Leslie Nielsen, and Sergeant Major Dagineau, Telly Savalas, a man whose harsh discipline has earned him a fearsome reputation. Dagineau is relentless in his pursuit of control, and he suspects Beau of being the author of an anonymous letter that has threatened his life. To root out the truth, he enlists Boldini, Malachi Throne, a pliant backslider who has reenlisted in the Legion and is promoted to Corporal to spy on the men.
Beau’s past becomes central to the drama. He ran away to join the Legion after taking a false blame for embezzlement that actually belonged to his business partner, a sacrifice made to protect the partner’s wife—whom Beau loves. The partner later confesses and commits suicide, a turn of events that haunts Beau and fuels the choice to enlist for five long years, even if it means never returning to the life he once imagined. Beau recounts this to his younger brother John Geste, Doug McClure, who has tracked him down with the same news.
On the march toward relief, De Ruse briefs the column that they are to relieve Fort Zinderneuf. But en route, the detachment is ambushed by Tuareg raiders, and De Ruse is mortally wounded by the time they reach the fort. Back in command, Dagineau reasserts his brutal rule, and the legionnaires mutiny, with everyone except Beau and John Geste prepared to execute the sergeant. Before the mutiny can unfold completely, the Tuareg renew their attack, forcing the issue into a larger fight.
Beau persuades the men to release Dagineau so he can lead them in defending the fort, despite their animosity toward him. As the siege rages, legionnaires fall under the relentless assaults, and Dagineau has the fallen lined up on the ramparts with rifles pointed at their attackers as a grim message to the enemy. In the quiet between assaults, De Ruse confides privately with Beau, admitting that he wrote the dangerous letter in a misguided attempt to provoke Dagineau to show more humanity toward his troops. Yet De Ruse’s confession only deepens the tragedy, for Dagineau’s brutal leadership grows harsher in response to the supposed threat.
Relief finally appears, but Dagineau delays the entry of the relief column to settle accounts with Beau. He insists the Legion needs heroes and that the dead on the walls can stand in as those heroes, so long as the truth of the mutiny remains buried. Beau’s confrontation with Dagineau ends in a deadly duel, and Beau kills the cruel sergeant, preventing more bloodshed and breaking the immediate threat to the fort.
The flashback returns to the present, where Beau has not yet told the relief commander the full truth of what happened. The commander presses for the real story, but Beau replies only that the men laid down their lives to protect the fort. With the higher command’s decision that Fort Zinderneuf is no longer worth defending, the relief is withdrawn, and Beau—having lost an arm in the ordeal—will be discharged. The final glance is quietly hopeful: Beau smiles at the idea that he truly has someone to return to, suggesting a future beyond the walls of the fort.
