In a distant future, the city of Metro City hovers above the Earth, sustained by advanced technology and populated by robots fulfilling various tasks. However, when a robot is deemed obsolete, it is discarded over the edge, landing on the chaotic landscape far below.
Amidst this high-tech world, we meet Toby Tenma, a brilliant young boy who visits his father, Dr. Tenma, at work. During his visit, he overhears discussions with President Stone about the creation of a Peacekeeping robot, a project aimed at securing re-election for the President. Dr. Elefun reveals that he has synthesized a powerful red core from a fallen star, which they intend to use in the robot. Unfortunately, this core turns the Peacekeeping robot into a malevolent force, leading to a cataclysmic event that vaporizes Toby.
In the wake of this tragedy, a heartbroken Dr. Tenma, with the assistance of Dr. Elefun, constructs a robotic version of Toby powered by a blue energy core. However, due to the nature of this creation, he cannot truly call the robot his son, prompting Elefun to release it into the world. Meanwhile, President Stone, driven by ambition, orders the capture of the new Toby upon spotting him soaring through the city. In a desperate attempt to seize the blue core, he inadvertently sends Toby tumbling from Metro City to the ground below.
There, Toby stumbles upon a group of children foraging for discarded robot parts. These kids, led by a girl named Cora, take him in, referring to him as "Astro." They warn him about Hamegg, a sinister figure who is notorious for destroying robots. Astro learns that Hamegg runs an orphanage where he exploits robots for underground fights, known as the robot games.
Cora reveals her own sorrow over being abandoned by her parents, while Hamegg manipulates Astro into reviving a massive construction robot, ZOG, with the blue core. However, things take an ominous turn when Hamegg, knowing Astro's true nature, forces him to fight in the robot wars against ZOG. Despite the challenges, Astro's bravery shines through as he helps ZOG resist Hamegg's coercion.
In a moment of compassion, Astro intervenes during a fight, saving Hamegg, which draws the attention of Stone's men, leading to his capture. Dr. Tenma contemplates deactivating Astro, but ultimately decides against it, allowing him to escape. In retaliation, Stone injects the red core into the Peacekeeper robot, activating a catastrophic transformation.
The Peacekeeper begins to assimilate other robots, swelling with power, as it wreaks havoc in Metro City. Astro rises to the challenge, attempting to confront the looming threat, yet he finds himself outmatched. Just when it seems all hope is lost, his friends, including Cora and the rest of the kids, come to his rescue.
As the giant robot causes Metro City’s power supply to fail, the city begins its descent toward Earth. In a selfless act, Astro enters the fray once more, supporting the city's weight to ensure a gentle landing. He realizes the only way to defeat the colossal threat is to merge the red and blue cores, fully aware that it would cost him his life. With determination, Astro dives into the enemy, merging the cores and destroying the monstrous robot, sacrificing himself in the process.
Devastated by this loss, Dr. Tenma mourns his creation, but the unexpected happens when ZOG uses the energy from Astro's blue core to revive him. The story concludes on a hopeful note, with the characters celebrating their victory and the return of their beloved friend, Astro.
