Koji (Alcor in Goldorak), pilot of Mazinger Z, and Tetsuya, pilot of Great Mazinger, are Japan's main weapons against the invasions of mechanical monsters developed by Dr. Hell, a megalomaniac bent on world conquest. Mazinger Z is treacherously stolen and Great Mazinger seems out of date: but Koji discovers a posthumous message from his grandfather, which allows him to discover a new robot, more powerful than all the others, Mazinkaiser. Koji struggles to take control of this infinitely powerful robot and manages to defeat the armies of Doctor Hell.
Partially adapted from Go Nagai's original manga, Mazinkaiser differs from Mazinger Z in that its violence is more pronounced and its humour is aimed at an adult and teenage audience.
The great success of the OVA in Japan led to a sequel, Mazinkaiser vs the Great Darkness General (2002), a one-episode 56-minute OVA that sees Mazinkaiser confront the underground empire of Mykene: this medium-length film was a synthesis of the Great Mazinger series.