When alien invaders from the planet Zela arrive, Earth's only defense is the super robot Gaiking and the Great Space Dragon.

The planet Zela is being consumed by a black hole, this phenomenon not only destroying the world beyond repair but mutating its citizens into fearsome creatures. Thus the Emperor Darius and his four generals assemble the Dark Horror Army, and head to conquer and enslave Earth as a worthy replacement for Zela. They take great care to assassinate any human with psychic powers who could serve as a threat to them, and succeed... nearly. Sanshiro Tsuwabuki, a baseball star whose powers were present but remained dormant, survived an attack that cost him his career. Now, he has been chosen to pilot Earth's one weapon against the Zeran army: a super robot named Gaiking. Accompanying Gaiking is the Daikyu Maru, a dragon-shaped carrier that can combine with Gaiking for more power. Together the Gaiking and the Space Dragon fight to protect Earth from the invading Dark Horror Army.—Tenchi