Many years in the future, after centuries of pollution and deforestation, the scientists of Metro City make the decision to spare their city this fate by removing it from Earth. Now, as the film begins, Metro City floats high above the Earth's surface, its human population tended by a massive robotic workforce who happily perform all the tasks humans no longer want to.
Finishing a pop quiz ahead of all his classmates, student Toby Tenma decides to visit the city's Ministry of Science, to see his workaholic father, the head of the ministry, Doctor Tenma. He arrives just as Tenma is meeting with the city's militaristic president Stone, to view a lecture be held by Doctor Elefun on a new energy source the ministry has discovered. From the heart of an ancient star, the ministry's scientsts have extracted pure, self-sustaining positive "blue core" energy, unfortunately leaving a destructive, negative "red core" byproduct. Stone is intent on using the powerful energies to fuel his new "Peacekeeper" military robot, and insists on installing the red core in the robot. Predictably, the negative energies of the core cause the robot to run amok, and it kills Toby before Doctor Elefun is able to use the blue core energy to cause an explosive reaction to shut down the Peacekeeper.
Driven to the brink of madness by the loss of his son, Doctor Tenma tries to replace Toby with a highly-advanced robot crafted in his image, programmed with all his memories taken from Toby's DNA and powered by the blue core, supplied by a reluctant but sympathetic Doctor Elefun. Once activated, the robot seems to be a perfect copy of Toby, but when Doctor Tenma takes him home, intending to finally spend time with the robot the way he failed to with Toby, he soon begins to notice differences. Tenma soon comes to realize that the robot is not the same as Toby, and it only serve to remind him of the son he lost, who he can never replace.
While in his room, "Toby" discovers that he can understand the electronic language of a pair of cleaning robots outside, and accidentally falls out of his window when he quarrels with them. In mid-plummet, he is saved by the inadvertent activation of a pair of jets built into his legs. Unfortunately, this causes a flare of blue core energy, attracting the attention of President Stone and prompting him to dispatch his troops to capture Toby and his core. Toby evades the troops and returns home, where Doctors Tenma and Elefun reveal his true robotic nature to him. Tenma tells Toby that he no longer wants him, and the devastated Toby flies off, but not before Elefun assures him that he has a place somewhere in the world. Mere moments later, however, Toby is ambushed by Stone's military forces, and sent tumbling to the Earth's surface far below.
Awakening amongst a pile of broken, discarded robots, Toby meets a robot dog named Trashcan, who leads him into a trap set by a gang of kids to capture robots. Mistakenly thinking Toby is a human, the kids set him free, but he is then suddenly abducted by a trio of robots who consider themselves the "Robot Liberation Front", who name him "Astro". The three robots warn the newly-christened Astro about the cruel Hamegg, who they claim enslaves robots, at which point the kids then arrive to rescue Astro and take him back to their home. There, Astro discovers that Hamegg is actually the children's Fagin-like father figure, a former inhabitant of Metro City who was fire from the Ministry of Science for "intimidating brilliance". Hamegg does not appear to be the evil figure the Robot Liberation Front claimed; instead, he repairs robots.
The next day, Astro (stilling hiding his robot nature from the kids, despite Trashcan's desire for him to share the truth) joins the other kids in searching for broken robots for Hamegg to fix, and comes upon a 100-year-old construction robot named Zog. Using the power of his blue core, Astro revives Zog, and the kids take him home and fix him up so he can partake in Hamegg's "Robot Games." Alas, it soon transpires that these "games" are anything but: they are actually gladiatorial matches in which the losing robot is destroyed. Having deduced that Astro is a robot, Hamegg puts him in the games as well, and Astro prevails until he is matched against Zog and refuses to fight him. Luckily, the positive influence of Astro's blue core has given Zog a life of his own, and he too will not fight. Zog turns on Hamegg and almost kills him (capable of doing so because he predates the Three Laws of Robotics), but Astro stops him. At that point, Stone's military forces arrive, having finally tracked down Astro, and capture him. Having seen the horrified faces of the children he thought were his friends upon discovering his was truly a robot, Astro does not fight back, and returns to Metro City with Stone. Stone mockingly offers his prisoner a "drink" of machine oil.
Stone brings Astro to Doctor Tenma, who has agreed to remove the core and place it within Stone's Peacekeeper. At the last moment, Tenma realizes that while Astro made not be Toby, he is still his son, and he allows Astro to escape. Intent on capturing Astro, the demented Stone once again places the red core in the Peacekeeper, and the robot again goes on a rampage, this time using its assimilating powers to absorb Stone himself. With his consciousness now driving the robot, Stone heads after Astro, absorbing every weapon and structure it comes across. In the resulting showdown, the power source keeping the city aloft is destroyed, but Astro manages to support it and set it down safely before continuing battle with Stone. Stone attempts to absorb Astro, but their diametrically-opposed cores react, and blast them apart. Knowing that the Peacekeeper must be stopped, even if it means sacrificing himself, Astro flies into the Peacekeeper's red core, resulting in a massive explosion that completely destroys the Peacekeeper, leaving Stone unharmed but arrested, and Astro dying since the blue core was drained in the explosion. However, because Zog was revived with the blue core, he is able to return some of the energy back to Astro, reactivating him.
Astro finds his place as a hero and everyone rejoices, but before any celebration can take place, a large alien attacks the city and Astro, now at peace with his robotic nature and his destiny as a hero, immediately launches into action after reassuring his concerned father, "I was made ready!"
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