Yuuma Azawa is a transfer student with an odd habit: he cannot stop apologizing. Taking pity on him, Keiko Arisu, his new classmate, insists he should stop, but all Yuuma responds with is a string of apologies. The bizarre encounters continue when Keiko notices a strange girl following her, eerily proclaiming that she wants to suck her brains out. Running in fear, Keiko jumps in a truck's way and ends up hospitalized. Yuuma pays Keiko a visit in the hospital and confesses that the girl she met is his younger sister, Chizumi. Befriending Chizumi, Keiko discovers Yuuma's dark past: he once summoned the devil and became obsessed, profusely apologizing as a form of a wicked offering. Still freaked out, Keiko returns to school; however, everyone’s brains terrifyingly melt out of their skulls—