“I finally decide to go back to school, and this is what I get?!”

After suffering from a severe, unknown illness for most of his childhood, eighteen-year-old Takahashi Haruki finally recovers enough to return to his high school— but as a first year. He is determined to get through high school peacefully until death takes him once and for all.

However, it seems fate considers Haruki’s hope for peace a joke, for a minor incident triggers a series of events that ends with him and his little sister, Mirei, transported outside of Japan! It takes several face slaps, but there’s no doubt about it: Haruki is in another world!

Even worse, their arrival inadvertently attracted the attention of a being more powerful than most. This strange person(?) with claws for hands and a perpetual frown will not let Haruki out of his sight. Haruki has no choice but to live alongside him, but is it wise to put his safety in the hands of someone whose personality changes like the phases of the moon?