Kappei is a kid of the Jin, a family of fishermen living in a small village. When alien invaders come to Earth, they reveal their alien nature. The Jin acestors came to Earth (and in Japan) during the Edo age, from planet Biar, after it was conquered by the army of Gaizok, who is now aiming at conquering Earth too.
With the help of ancient documents, they manage to find the ships of their ancestors, with the help of which they are able to fight their old enemy, by assembling the robot "Zambot 3".