Dr. Hell and his Mechanical Beasts have returned, proving more dangerous than ever. Kouji Kabuto and Tetsuya Tsurugi engage in a fierce battle against these hellish machines, which operate under the command of Baron Ashura. Unfortunately, they are defeated; Kouji's Mazinger Z is captured, and Tetsuya's Great Mazinger is seriously damaged by the enemy. The devious Dr. Hell quickly converts the captured robot into the Ashura Mazinger and uses it to attack the Photo Power Lab. Meanwhile, Kouji is missing in action, leaving Tetsuya to defend the lab in the damaged Great Mazinger. However, the forces of Dr. Hell are too strong, even for a brave pilot like Tetsuya, and all seems lost. The battle appears to be drawing to a close, until an overpowering blast fired by an unknown robot destroys the entire Mechanical Beast army. Could this mysterious robot be the legendary Mazinkaiser, and who is piloting it?