Mazinkaiser is an OAV re-imagining of the Mazinger Z story that was released in 2001. It tells the story of how Kouji Kabuto discovers and masters his grandfather's final legacy, the most powerful of the Mazingers.
Mazinkaiser originally appeared in Super Robot Wars F Final, when Banpresto asked Go Nagai to design a new mecha for Kouji to use as a Mid-Season Upgrade. The idea was for Mazinger Z being upgraded through the exposure of Getter Rays into a new monstrous form, later redubbing it the Mazinkaiser. The idea was such a hit with fans that Go Nagai quickly incorporated it into Mazinger canon in anime and manga form. During a massive attack by Mad Scientist Dr. Hell's mechanical beasts, the Mazinger Z and Great Mazinger both sortie to battle, but are eventually overrun through sheer numbers. Kouji's Hover Pilder is ripped from Mazinger Z and hurled from the battlefield, and Mazinger Z itself is captured by Dr. Hell, who turns it into a mechanical devil for his own purposes and sends it against the Photonic Research Institute. Outnumbered, Tetsuya Tsurugi goes to battle with a damaged Great Mazinger when suddenly a new, insanely powerful and more demonic looking Mazinger shows up with Kouji at the helm...
The OAV ran for seven episodes, and was followed up by the movie Mazinkaiser VS the Great General of Darkness, where the Mycenae Empire invades while Kouji is vacationing in Paris, forcing him to fight his way back to Japan and take Mazinkaiser into battle once more.
There's also a second OAV series, Mazinkaiser SKL. This particular take is a very different re-imagining, with all-new characters. It looks a Mazinkaiser-meets-Super Robot Wars Gaiden-meets-Super Robot Wars Compact 3 mix. We're not sure how that will work out, but It looks pretty damn cool.