The series begins after the battle against Dr. Hell with attacks from the Warrior Beasts. With the Mazinger Z overwhelmed by this new threat, it was almost destroyed until it was rescued by the Great Mazinger. With assistance from the Science Fortress Laboratory, Tetsuya Tsurugi and the Great Mazinger battle against the risen Mycenae Empire. With the defeat of military leader, Great General of Darkness peace had came for once a brief while. The Empire brought Dr. Hell back from the dead as the new military commander and overpowered the Great Mazinger until the restored and strengthened Mazinger Z came back to fight. Koji and Tetsuya would fight together to bring down this threat with their robots and their allies. With a final assault on the empire, the Mazinger brothers emerge victorious.
—http://mazinger.wikia.com/