The OVA begins with the Mazinger Z and Great Mazinger fighting against Dr. Hell's Mechanical Beasts along with the Aphrodite A and Boss Borot. While Koji believes the Mazinger brothers can handle, the robots are soon overpowered and Koji is launched from the Mazinger Z with the Hover Pilder going off in some direction. Upon landing he uncovers the Mazinkaiser, a powerful robot secretly created by his grandfather after finishing Z. As the hologram of Juzo disappears after boasting that the Kaiser can give Koji the power to exceed a god and conquer the devil, Koji goes to cockpit but loses consciousness as it fights off the Z controlled by Baron Ashura and almost destroys the Photon Power Lab. After Tetsuya and Jun leave, Koji tries to gain complete control over the Kaiser while battling Dr. Hell.
Mazinkaiser was a success during its release, even getting an adaptation in English a few years later. Its success brought on the release of Mazinkaiser vs. Great General of Darkness.