The original Japanese name for the series has been translated literally as Giant Robot.
Guillotine and his henchmen arrive on Earth in a spaceship which hides at the bottom the the ocean (near Tokyo of course). From this underground base Guillotine oversees his evil schemes to cause havoc and destroy the world using a variety of giant monsters. To help him in this he kidnaps Dr. Lucius Guardian and forces him to build a huge, powerful robot - Giant Robo.
Before the robot can be put into action, Guillotine is discovered by the young Johnny Sokko and his friend Jerry Mano. Mano is actually an undercover agent for Unicorn, a secret organisation who are the Good Guys in the show. They obtain control of Giant Robo which from then on responds only to commands from Johnny Sokko. With this new power at his disposal, Johnny is invited to join Unicorn. I wonder what would have happened had he declned?
Giant Robo has a number of powers, but essentially it's a stonking great metal robot that can fly! Its powerful array of weapons includes missiles, eye beams and a high voltage electrical wire.