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.
(Source: The Anime Encyclopedia)