Amnesiac Caiman seeks to undo his lizard head curse by killing the sorcerer responsible, with his friend Nikaido's help. In the Hole, that's a threat.

Hole-a dark, decrepit, and disorderly district where the strong prey on the weak and death is an ordinary occurrence-is all but befitting of the name given to it. A realm separated from law and ethics, it is a testing ground to the magic users who dominate it. As a race occupying the highest rungs of their society, the magic users think of the denizens of Hole as no more than insects. Murdered, mutilated, and made experiments without a second thought, the powerless Hole dwellers litter the halls of Hole's hospital on a daily basis. Possessing free access to and from the cesspool, and with little challenge to their authority, the magic users appear indomitable to most-aside for a few. Caiman, more reptile than man, is one such individual. He hunts them on a heedless quest for answers with only a trusted pair of bayonets and his immunity to magic. Cursed by his appearance and tormented by nightmares, magic users are his only clue to restoring his life to normal. With his biggest obstacle being his stomach, his female companion Nikaidou, who runs the restaurant Hungry Bug, is his greatest ally.—MAL Rewrite

The city of Hole is populated by regular humans. However, it is also a popular tourist destination for magic users looking to practice their spells on the innocent. Kaiman is a human who has lost his memory and had his head transformed into a reptile's. In order to get his head back and recover his memories, Kaiman decides to hunt down the magic user who made him this way.—hexedghost