Grade schooler Yuuta Tomonaga stumbles upon Deckerd, a humanoid robot under construction by the Japanese police, built to fight advanced forms of crime. Yuuta's constant contact with Deckerd gives the robot a "heart," or personality; when Yuuta is recruited as the "boss" of the "Brave Police" as a result, a true human/robot partnership occurs.

A great war occurred in the 20th century between humans and monsters. Since that time, the monsters rule the world in fear but keep relatively hidden from public view. A boy named Bokka ponders the past and wonders what became of the Meros Warriors who defended the world so bravely against the demons. He soon meets Kurofune and learns of the power of the Meros and the I-bar machines they ride in battle. Warriors are the only ones who can see and hear the Melody of Oblivion, a phantom girl hidden away waiting to be rescued and be the savior of mankind. During a battle between Kurofune and a demon, Bokka discovers something only his wildest dreams could possibly imagine...he too can hear that melody. Throughout his journeys, Bokka meets many monsters and their agents, friends, and companions as he discovers the true extent of his new powers. He must continue to battle evil in the hope of releasing Boukyaku no Senritsu and free a world that has forgotten its once beautiful melody.

Dr. Hell and his Mechanical Beasts are back, and they're more dangerous than ever before. Kouji Kabuto and Tetsuya Tsurugi fight a fierce battle against these hellish machines, which are under the command of Baron Ashura. Unfortunately, they are defeated, with Kouji's Mazinger Z being captured and Tetsuya's Great Mazinger seriously damaged by the enemy. The devious Dr. Hell quickly converts the captured robot into Ashura Mazinger and uses it to attack the Photo Power Lab. At the same time, Kouji is missing in action, which leaves Testuya to defend the lab in the damaged Great Mazinger. But the forces of Dr. Hell are too strong, even for a brave pilot such as Tetsuya, and all seems to be lost. The battle seems 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?

This is the story of Mito Mitsukuni in mecha style. Prince of the Edon Star, Edward Mito begins a trip to the galaxy with his servant Baron Kalkus, Duke Skead and the ninja girl Shinobu. Under the false identity of Bocchan, Suke and Kaku they help the oppressed people and fight with the super robot Daioja.