Dr. Hell and his fleet of Mechanical Beasts have returned, more dangerous than ever before. Kouji Kabuto and Tetsuya Tsurugi engage in a fierce battle against the machines, which are under the command of the nefarious Baron Ashura. Unfortunately, both pilots are defeated, with Kouji's Mazinger Z being captured and Tetsuya's Great Mazinger being severely damaged. Dr. Hell quickly converts the captured robot into Ashura Mazinger and orders it to attack the Photo Power Lab. Meanwhile, with Kouji missing, Tetsuya bravely defends the lab using the battered Great Mazinger. Despite his determination, Tetsuya is no match for Dr. Hell's forces, and a defeat seems inevitable. As the battle nears its end, an unknown robot unleashes a powerful blast that annihilates the entire Mechanical Beast army. Could this robot be the legendary Mazinkaiser? And who is behind the controls?