Shortly after a group of aliens secretly land on the Earth, they kidnap Dr. Lucas Gardion, a brilliant scientist, and force him to construct Giant Robot, an indestructible mechanical machine that they plan to use to destroy Earth. As the doctor completes the robot, a prehistoric type of creature brought to Earth by the aliens escapes and attacks a Japanese ship. Two people, Johnny Sokko (a young boy), and Jerry Mono, an agent for Unicorn (an international defense organization), survive and wash up on a nearby uncharted island -- the alien's base.

There, they meet Dr. Gardion, who tells them about the robot and what the aliens plan to do. They also learn that the robot will obey the first voice it hears and that it is complete except for an atomic charge to activate its circuits. Just then, as the aliens attack the lab, Dr. Gardion detonates the atomic bomb he planted in the robot to destroy it. However, instead of destroying it, the bomb supplies the necessary power to activate it.

Amid the ensuing confusion, Johnny finds the wristwatch- type control and speaks to the robot. He becomes its master. With the help of Giant Robot, Johnny and Jerry escape and return to Tokyo (where the series is set) and where Johnny becomes a Unicorn agent and Giant Robot becomes Earth's most valuable defense weapon. The series depicts Unicorn's battle against sinister alien invaders. Produced in Japan.