During World War II, SS Lieutenant Reinhardt Kirk B.R. Woller arrives at the Citadel, a French castle repurposed as a German laboratory, and relieves Professor Ullman Ben Cross of command. He discovers three caged dogs that are horribly mutated—extremely muscled, their bodies glowing—while Ullman explains these are his latest experiments and eagerly requests one of Reinhardt’s men for a demonstration. In a capsule, a soldier is injected with a chemical and exposed to an unknown type of radiation; within moments he collapses, transforms into a blue-skinned, massively muscular creature with clawed limbs, and the Doomtrooper is born.
Across the Atlantic, Captain Pete Malloy Corin Nemec is tasked with locating and destroying the Citadel. He forms a team of British and American prisoners who are offered their freedom in exchange for participation, and he also enlists his Royal Marines friend, Sergeant Digger Harry Van Gorkum. They make a harrowing, early parachute drop into German territory under heavy anti-aircraft fire. Two soldiers, Corporal Johnson and Parker Lewis Private Parker Lewis, are separated from the main group and pushed toward the meeting point.
Lieutenant Reinhardt, learning of the Allied arrival, authorizes Ullman to test the Doomtrooper in earnest. Johnson and Papadakis (Private Andy Papadakis Assen Blatechki) stumble upon the creature in a local village. They fire at it, but the Doomtrooper shrugs off bullets; Johnson is knocked aside by a machine gun mounted on the being’s arm. Papadakis prepares to fight, but the rest of the team arrives and drives the thing back long enough for Papadakis to escape. The group believes the monster is dead after tossing grenades, though the threat obviously isn’t finished.
Reinhardt is thrilled by the report of a powerful foe facing the Allied soldiers and orders a German team to recover the Doomtrooper. Ullman confides in his assistant that the soldiers will be killed by it, and that prediction proves true when the Doomtrooper slaughters the German squad and electrocutes its leader.
The Allied soldiers seek shelter and unknowingly stumble into a French Resistance hideout led by Mariette Martinet Marian Filali. They decide to team up, pooling forces to strike a plan: raid a nearby ammunition dump and use the explosives to destroy the Citadel once and for all.
That night the Doomtrooper attacks the hideout, slaughtering many of the Resistance fighters. Corporal Potter briefly disables it with a grenade launcher, but when the creature recovers it grabs Potter and hacks him to death. Private Lewis twists his ankle in the escape, nearly kills himself, and the Doomtrooper, running low on ammunition, loses track of the remaining attackers.
Back at the Citadel, Reinhardt orders Ullman back to Berlin and signals an end to the Doomtrooper project. Enraged, Ullman shoots Reinhardt and his adjutant, then orders his assistant to gather more men so he can breed new Doomtroopers.
On the way to the ammo dump, the team encounters a German panzer out hunting for the Doomtrooper. Papadakis kills the tank commander and they seize the tank. The Doomtrooper attacks, and a point-blank tank shot stuns it but does not stop it. Jones disguises himself as the tank commander, guides the team to the ammo dump, and draws German guards away. They seize a guard post with a machine gun and blow the dump to pieces, hoping to annihilate the Doomtrooper.
Malloy and Mariette lure the Doomtrooper into an ammunition bunker using a flamethrower, then escape by elevator while the rest of the team seals the bunker door, imprisoning the creature. Jean-Michele Julian Bailey attempts to bar the door, but the Doomtrooper electroshocks him through the iron barrier. The others retreat to a safe distance and detonate the munitions, decimating the threat.
The survivors press on to the Citadel, but the Germans are ready. They encounter a sniper, and Jones is shot while trying to provide cover fire; Malloy must decide whether to save him, but Jones’s family medallion has deflected the earlier bullet, revealing a glimmer of luck in the fog of war. Jones does not survive the encounter, but his sacrifice buys Malloy time to continue.
Jones reappears moments later with a rocket launcher, leading the defenders in a diversion that allows the rest of the team to slip into the castle. They kill the guards and confront Ullman in the lab, where Ullman’s assistant is killed by the awakened Doomtrooper. Ullman orders the Doomtrooper to kill everyone in the room, so Malloy fights it briefly, cutting off its hand to slow it down, though bullets and missiles have little effect.
Lewis manages to short-circuit the lab’s power, but is electrocuted in the process. Malloy lunges, tearing free, and jams two electrical cables under the Doomtrooper’s helmet, delivering a fatal jolt. The creature collapses, and Malloy, Mariette, and a still-alive Digger barely escape as the castle crumbles around them.
Back in America, Captain Malloy reports to General Carmichael. When asked for any experimental data, Malloy says the project killed his friends and should never be recreated. As they drive away, he quips that the General wouldn’t mind if they borrow his car, and the trio disappear into the night, leaving behind a cautionary tale of ambition and ruin.
