In one of their epic battles, the Mazinger Team suffers a crushing defeat at the hands of Dr. Hell and his minions after Mazinger Z is stolen and its pilot, the legendary Kouji Kabuto, is rendered missing in action. Just when Dr. Hell's second-in-command Baron Ashura uses Mazinger Z against our heroes, Kouji makes a surprise return in Mazinkaiser - an all-new Mazinger with heavier armor and deadlier attacks.
 
(Source: Anime News Network)

They strike without warning and instantly plunge the world into terror. The Great General of Darkness leads the underground Mycenae Empire in a bid to conquer Earth. The Mazinger Army falls before the seven generals under him, each of them leading an army of superior Warrior Beasts. Meanwhile, Archduke Gorgon is relentlessly pursuing Koji Kabuto, trying to eliminate him before he can reach Mazinkaiser. But a red light in the sky over Mt. Fuji portends the greatest battle in history!
 
(Source: Discotek)

From the island of Bardos in the Aegean Sea comes a new threat to humanity in the form of Doctor Hell and his army of mechanical beasts. However, a new hero stands in his way: Mazinger Z, a giant robot built out of the indestructible metal chogokin and equipped with an arsenal of advanced weaponry. Piloting Mazinger Z is hotheaded teenager Kouji Kabuto, who strives to avenge the death of his grandfather and protect Japan from Doctor Hell's forces.
 
(Source: Anime News Network)

Prior to the events of the anime, three factions - the Garan Army, the Kiba Army, and the Aira Army, battle for total dominance of Machine Island. The island is basically a prison, isolated from the rest of the world. The island contains a vast amount of resources, allowing the three factions to create an infinite supply of giant robots for their respective armies. 
The world government learns that the island's power reactor has become unstable and will explode within 66 hours, destroying all of life and Earth itself. The government sends the Death Caprice squad, composed of mercenaries Ken Kaido and Ryo Magami to solve the issue, along with their giant robot Mazinkaiser, but was that a good idea?

Crossover one-shot between Mazinkaiser and Shin Getter Robo. It was included in Immortal Super Robot Encyclopedia (不滅のスーパーロボット大全).
The manga begins with the Mazinger Z and Great Mazinger fighting against an alien machine after the Diana A and Venus A have been taken down.
(Source: Mazinger Wikia)

Slight retelling of the 2001-2002 Mazinkaiser OVA series.

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