>>283268550
The first chapter was kind of messy. It's like a battle royale where the protagonist (Shiren) suddenly wakes in the "Mazinger Zest" and then is forced to fight against other Mazingers, with some flashbacks to his previous life. Then after time runs out, he's taken to a dock and prepared to the next battle. He's in this situation for the entire manga with some reveals about the outside world through exposition later on.

Half of the series has him fighting small variations of Mazinger Z, although in the 2nd half they start referencing other Nagai stuff, like there's a Groizer and a robot based on Sirene from Devil Man. The Great Mazinger-like robot also only appears by this point.