Mechagodzilla is a robotic duplicate of Godzilla.
Mechagodzilla was a giant robot that was created by aliens to destroy Tokyo so they could build their new civilization there (they were doing this because their planet was being swallowed by a black hole). Constructed from the powerful metal space titanium, Mechagodzilla was brimming from head to toe (literally) with weapons of all sorts. His armaments included eye lasers, a mega-ray in his chest, finger-missiles, knee-missiles, and toe-missiles. Mechagodzilla's first fight was with Anguirus, whom he thoroughly trashed, who once beaten most likely alerted Godzilla about Mechagodzilla.
After this, it was only a short while before Godzilla showed up and fought Mechagodzilla, but in the battle both monsters were injured and needed healing/repairs. After he returned he wiped the floor with King Caesar and was ready to kill him when Godzilla showed up. After a short thrashing Godzilla suddenly developed magnetic powers and soon ripped off Mechagodzilla's head.
Later, the same race of aliens found and rebuilt Mechagodzilla to once again destroy everything. This time, with the help of Titanosaurus, Godzilla was thoroughly trashed, until agents of Interpol interrupted Titanosaurus' ears with supersonic transmissions. Godzilla gained the advantage, and once again ripped off Mechagodzilla's head, only to find that the aliens had this time given him a back-up head. Thanks to the intervention of Akira Ichinose and Jiro Murakoshi, who destroyed the alien controls, Godzilla finally managed to put a stop to the threat of Mechagodzilla.