Yuuma Azawa is a new student with a strange habit that he can't quit: apologizing without end. Despite classmate Keiko Arisu's attempts to intervene, Yuuma continues to apologize. Suddenly, Keiko is stalked by a frightening girl who threatens to devour her brain. In a panic, Keiko runs into traffic and is badly injured.
In the hospital, Yuuma visits Keiko and confesses that the girl following her is his younger sister, Chizumi. As Keiko strikes up a friendship with Chizumi, she uncovers Yuuma's disturbing past: he once summoned a devil and became consumed by the idea, using his apologies as a form of sacrifice.
Although terrified, Keiko returns to school, only to witness people's brains liquefying out of their skulls in a macabre display. Chizumi claims that this is due to Yuuma's never-ending apologies. Keiko barely avoids the same fate, but the tragedies that befall those around the demonic siblings grow worse and more frequent.
Included in this manga is a one-shot called "Saikai, Daichi no Ko."
