An army of machines, led by Baron Ashura, defeats Kouji's Mazinger Z and Tetsuya's Great Mazinger. In the aftermath, Kouji stumbles upon a forgotten laboratory. Inside, he finds Mazinkaiser, the most powerful robot ever built. The series chronicles Kouji's experience with the machine as he copes with Kaiser's power while defeating Dr. Hell's forces.
The OVA's design is inspired by the original Mazinger Z manga from 1972, rather than Toei Animation's TV series. Mazinkaiser emulates Go Nagai's character designs and its operatic plot is supported by a "tongue-in-cheek" cynicism in regards to the gratuitous sex and violence.
An English language version was released by ADV Films in 2003.
Episode 2: A Descending, Malevolent Deity: Great Mazinger attempts to battle the mystery robot, but is easily swept aside. Just as the machine begins to power up its heat sinks, Sayaka arrives and begs the pilot to stop. Kouji later awakens in bed, having little recollection of piloting Mazinkaiser to the rescue and attacking the lab himself. He describes how he found Mazinkaiser, the final creation of his grandfather. Unfortunately, Mazinkaiser taps into the pilot's mind and is so powerful Kouji is unable to use its power effectively, or even safely. Because of this, when Ashura uses the aerial fortress Gool and to attack a nearby city Great Mazinger, Aphrodite A and Boss Borot are dispatched while Kouji helplessly fumes back in the lab. Predictably, the three heroes are no match for the Mechanical Beast onslaught. When Kouji arrives, Ashura captures Sayaka and threatens to kill her if he does not surrender Mazinkaiser. Kouji separates the Kaiser Pilder from Kaiser itself, but as it is being loaded onto the ship the Great Booster arrives and cuts the chains, sending Ashura's forces into disarray as it barrels through them. Taking advantage, Kouji quickly yells "Pilder ON!" and brings Mazinkaiser to life. After a short but sharp battle, Ashura is sent packing. Tetsuya decides to leave with Jun to give his wounds time to heal and entrust the safety of Japan to Kouji.
Episode 3: Kouji Assassination Order!: Enraged with the appearance of Mazinkaiser, Dr. Hell is furious with Baron Ashura. However, the wily henchman informs his master that he has already dispatched his three cute daughters to deal with Kouji, since without Kouji to pilot it Mazinkaiser is useless. Ashura's daughters attack Boss by mistake, prompting Dr. Yumi and Sayaka to forbid Kouji from leaving the Lab until the situation changes. Hot-headed as ever, Kouji sneaks out anyway and battles the three daughters of Ashura, who turn out to be killer androids. He defeats them just in time to rescue Sayaka and Boss from a Mechanical Beast. The episode finishes with the introduction of Lori and Loru, two busty American sisters who are Dr. Yumi's assistants. This episode introduces the Mazinkaiser version of Venus A, piloted by Sayaka instead of Jun.
Episode 4: Sayaka Rescue Operation: On a day at the beach, Ashura kidnaps Sayaka and threatens to publicly humiliate her by cutting off her swimsuit if Kouji and Mazinkaiser do not surrender. Kouji is brought aboard the submarine Salude, but manages to escape and rescue Sayaka with Boss' help. Venus and Mazinkaiser make short work of the submarine. This episode is somewhat infamous for its crude sexual humour.
Episode 5: A Narrow Escape! The Photon Power Lab: Dr. Hell is fast losing patience with Ashura's failures, prompting the half-male, half-female villain to carry out a desperate plot. He attacks Dr. Yumi as he and Lori are travelling back to the Photon Power Lab and then disguises himself as the doctor. Unfortunately, he had no way of knowing it was the doctor's birthday and so was caught off-guard. Eventually losing patience, he attacks the Lab and is repelled. Dr. Hell angrily has him imprisoned and resolves to deal with Mazinkaiser himself. In the course of the episode Dr. Hell chances upon a group of more powerful, partly-organic giants (Mycene Warrior Beasts) which he utilizes in place of the previous mechanical monsters. This episode is notable for having a disturbing shower scene with Ashura.
Episode 6: Kouji Kabuto Dies in Magma!: Dr. Hell launches a full scale assault on the Photon Power Lab, using a Mechanical Beast to infect Venus A and turn it against them. His attack is followed up by a ground assault that successfully captures Kouji and the others. Boss and Boss Borot distract the enemy long enough for Kouji to reach and activate Mazinkaiser, but shortly thereafter Kouji is taken by surprise by the Combined Mechanical Beast (Gattai Kikaijuu). Unable to effectively combat the flight-capable Mechanical Beast, Kouji and Mazinkaiser are dumped into an active Mount Fuji.
Episode 7: A Decisive Battle!! Flaming Hell Castle!: After Mazinkaiser is dumped into Mount Fuji by the Combined Mechanical Beast, it pursues Boss, Sayaka and the other members of the Photon Power Lab. Before it can kill them it is struck by a massive lightning: the Great Hero, Great Mazinger has returned! While Great battles the Mechanical Beasts, Jun arrives and takes the group to the hidden fortress where Kouji discovered Mazinkaiser. She explains that she and Tetsuya discovered and explored the base, uncovering another Great Mazinger. This Great is far more powerful than the one Tetsuya piloted in the previous episodes and is described as Shin Great Mazinger (the true Great Mazinger). The original Great Mazinger was the prototype version made by Dr. Kenzo Kabuto. At that time, Dr. Juzo Kabuto, thought to be dead, was secretly making the final version of Great Mazinger in his underground laboratory. He was also able to finish Mazinkaiser as his ultimate masterpiece. They also uncovered Kaiser Scrander, the wings of Mazinkaiser. Unknown to the unconscious Koji at Mt. Fuji, his Pilder has activated an automatic homing signal, which will activate the Kaiser Scrander. Diving into Mount Fuji, the Scrander docks with Mazinkaiser and the now complete Mazinkaiser emerges from the volcano. After briefly assisting Great, Kouji decides to attack Dr. Hell's base, which was uncovered when he launched his entire Mechanical Beast army. Great Mazinger faces the Mechanical Beast horde as Kouji arrives on Dr. Hell's island and is confronted by Baron Ashura, now rebuilt into a Mechanical Beast. After a long and difficult battle, Kouji emerges triumphant thanks to Mazinkaiser's Final Blade and thoughts of his friends and family. Dr. Hell attempts to win Kouji over to his side and when the hero refuses activates the island's self destruct sequence. Unluckily for the villain, his escape craft is caught in the explosions as well, while Mazinkaiser emerges unharmed from the inferno.