From the TV series of "Astro Boy," three episodes were selected for theater re-edition, "Robot Rocket" (46th episode), "Earth Defense Troops" (56th episode) and "The Last Day of Earth" (71st episode). Of these three, "Earth Defense Troops" was the only one originally derived from Tezuka Osamu's story "Number 7." Its settings were diverted to the "Astro Boy" series. In this film, the whole scene of "Earth Defense Troops" and a part of "The Last Day of Earth" were reproduced in color for this theater version.

In the year 2003, Professor Tenma is distraught when his son Tobio is killed in a car accident. He loses himself in his latest project, creating Atom, a robot boy programmed to be forever good.

Upset that his Tobio-substitute can never grow up, Tenma sells Atom to Ham Egg, the cruel ringmaster of a robot circus. Atom meets the kindly Professor Ochanomizu, who adopts him, inspires him to become a crusader against evil, and eventually builds him a robot "sister," Uran.

Before producing a TV series, a pilot film was made to let sponsors and television stations grasp the work's pervasive quality. This is the pilot film for "Dororo" in which the characters are closer to those of the original story compared to the characters in the TV series. The faces and personalities of the characters in the TV series are altered from time to time to gain popularity among its viewers. As a result, sometimes viewers really loved it, but at other times it didn't go over as well. The pilot for "Dororo" was made in color, but the TV series was monochrome due to a tighter budget. The story depicts a journey in which the boy thief named Dororo and a cursed man called Hyakkimaru destroy monsters. Hyakkimaru can be a complete "human" only after he destroys 48 devils.

Hyakkimaru is a young man who lacks 48 body parts because they were taken from him by demons before birth, as payment by his father, Kagemitsu Daigo, to obtain his wish to take over the country. When the baby boy was born he was missing 48 parts of his body, and thus was abandoned—thrown into a river. Hyakkimaru has grown up and now has obtained fake body parts so he can eliminate the 48 demons that were made from his body, and to retrieve his missing parts. Along for the adventure is the boy thief, Dororo, with whom he becomes friends.