In the distant future, Metropolis — once Earth — has become a peaceful utopia under The Great Brother [Massimo Serato]. That serenity is suddenly threatened when his brother Graal [Ivan Rassimov] escapes a prison satellite, steals a warship, and allies with the mad scientist Dr. Kraspin [Arthur Kennedy] to forge an army of unstoppable supersoldiers called the “Humanoids.” The key to their mutation project is Kapitron, a potent element kept at the Grovan Institute on Metropolis.
Graal dispatches a raid team to seize the Kapitron and, if needed, to erase any obstacle in their path. Among their targets is Barbara Gibson, Barbara Gibson, [Corinne Cléry], a scientist who once worked with Kraspin but who recognized the danger of the element and had him confined years earlier. Barbara leaves before the slaughter when summoned home by her enigmatic pupil, a boy named Tom Tom, Tom Tom, [Marco Yeh], who displays powerful mental abilities and periodically communes with two mysterious travelers.
Meanwhile, Kraspin has found asylum on the distant world of Noxon, where he is sheltered by Lady Agatha, Lady Agatha [Barbara Bach], a tyrant queen who dreams of ruling the galaxy in a union with Graal. Kraspin supplies her with a youth serum—a macabre process that drains life from female slaves—to keep Agatha young and beautiful while he works his dark experiments. With Kapitron now in hand, Kraspin chooses his test subject: Golob, Golob [Richard Kiel], a bearded giant whose ship is forced to crash into a lake, where Kraspin fires a Kapitron missile that mutates him into a raging, superhuman hulking figure. Golob’s robot dog Kip watches the chaos as Kraspin and Graal abduct the behemoth for their plans.
A brain-control chip is implanted in Golob, and he is unleashed on Metropolis with orders to kill The Great Brother. Nick, a Metropolis security officer, tries to shield the ruler, but Golob’s overwhelming power drives him forward—until Kraspin suddenly shifts his orders to go after Barbara Gibson first. Golob departs, leaving Nick and the Great Brother to regroup.
Golob tracks Barbara to her home, but Tom Tom uses his powers to calm him and removes the control chip. Free of Kraspin’s grip, Golob becomes a staunch ally to Tom Tom and Barbara, explaining in vague terms that Graal and Kraspin are hiding on Noxon. Barbara resolves to warn Nick and The Great Brother, but in transit she is captured by Graal’s forces. Tom Tom and Golob stage a rescue but are forced to retreat when Graal’s soldiers open fire. They are saved at the last moment by the two travelers, who shoot glowing arrows and help them slip away.
Back at Metropolis’s command center, Nick devises a rescue plan for Barbara, but the plan is complicated by the ever-present threat of Noxon’s civilians. Golob volunteers to undertake the mission, and Nick joins him. Tom Tom stows away as a would-be ally, and the trio heads toward Noxon. Their ship is ambushed by Graal’s fighters and crash-lands near the base, where they meet Kip again and use the robot’s help to infiltrate Graal’s stronghold. Inside, Kraspin reveals a giant Kapitron missile intended to blank Metropolis with mutational energy.
Barbara endures a grim procedure in Kraspin’s laboratory, but she is rescued just in time when Nick and Golob burst in. As Agatha and Kraspin try to flee to Graal’s ship, the heroes launch a full assault on the launch bay. Kraspin is killed when Golob topples the Kapitron launch pad, while Agatha—unable to obtain her youth serum—rapidly ages into a skeleton, meeting her end. Nick pursues Graal onto the bridge of Graal’s ship and faces him in a deadly duel. Graal fires lasers from his hands and nearly takes Nick’s life, until Golob intervenes, crushing a control panel on Graal’s chest and leaving him to disappear into the void.
Golob then rips the Kapitron container from the missile, and the rocket—now weaponized by that stolen material—is hurled toward Graal’s ship, destroying it in a cataclysmic explosion. As the facility collapses around them, the heroes escape—though Golob initially seems lost in the chaos. Tom Tom spots him fleeing with the Kapitron, leaps into a lake to pursue, and the lake explodes behind them. The mutation’s effects reverse, and Golob surfaces, alive and restored enough to celebrate the group’s reunion.
A mysterious translucent sailing ship appears in the sky, piloted by the two travelers. Tom Tom explains that his mission is complete and that he must return to Tibet in the ancient past, but he promises Barbara that he will always be with her in her heart. With that, Tom Tom departs on the ship, a final image of departure and memory as Metropolis’s future hangs in the balance and a hard-won peace settles over the planet.
