In a futuristic world where robots are common, a brilliant but grief-stricken scientist named Dr. Tenma creates a powerful robot boy to replace his own son, Tobio, who died in a traffic accident. Built with immense strength, rocket-powered limbs, and advanced artificial intelligence, the robot is designed to look and act like a real human child. However, Dr. Tenma eventually becomes disappointed when his creation cannot grow or truly replicate the emotions of a lost son, leading him to abandon the robot.

The robot boy falls into the clutches of a cruel circus owner named Hamegg, who forces him to perform as a stagehand and exhibit, treating him harshly. His fortunes change when he is discovered by Professor Ochanomizu, the kind-hearted and wise head of the Ministry of Science. The professor rescues the robot, becomes his legal guardian, and gives him the name Astro Boy, helping him integrate into human society as a sixth-grade student. Professor Ochanomizu recognizes that Astro Boy possesses not only incredible powers but also a unique capacity for human emotion, known as Kokoro.

As Astro Boy adjusts to his new life, he uses his superpowers to protect Metro City from various threats. These include renegade robots, alien invaders, and human criminals who hate robots. A recurring theme is the social prejudice and discrimination that robots face from humans who view them as mere tools. Despite being a powerful superhero, Astro Boy is a pacifist at heart who seeks to solve problems through understanding and compassion rather than violence whenever possible. He is often joined by his spunky younger sister, Uran, another robot created by Professor Ochanomizu, and his human school friends Kenichi, Shibugaki, and Tamao.

The story explores Astro Boy’s search for his own identity as he grapples with the knowledge that he was created to replace a dead child. His original creator, Dr. Tenma, often reappears as a complex antagonist, sometimes seeking to reclaim his creation or prove that robots are superior to humans. Astro Boy also faces powerful robot rivals like Atlas, a similar creation built with a darker purpose, and must confront major conflicts such as a robot revolution led by the Blue Knight and anti-robot movements pushed by humans like the politician Acetylene Lamp. Throughout these adventures, the series consistently raises deep philosophical questions about the relationship between humanity and technology, the nature of consciousness, and what it truly means to have a heart.