Jimmy Sparks is puzzled to discover one day that Gigantor's movements aren't as effective as before. Dr. Brilliant examines the robot and finds out that it's a fake! Soon many Gigantors are sighted, leading to even more startling discoveries: Somebody is producing less effective replicas to hide the real one stolen right from Jimmy's complex. On top of that, the real Gigantor is under the control of Captain Spider and is now on its way to the Moon, where Spider is building a missile launch base so he can spray Earth with neutron bombs!