After his son Toby dies in an accident, Dr. Tenma builds a robot named Astro to replace him. Instead of becoming a member of the family, Astro is disowned because nothing can ever truly replace Tony.

Dismayed by the fact that his new son cannot grow, Tenma cruelly sells Astro to a greedy circus leader known as Hamegg. Shortly thereafter, he is rescued by the kindly Dr. Elefun and begins a new ...