High school girl Kanzaki Mitsuki "inherits" an older brother when her mother remarries a fellow divorcée with a son. Soon after, her new stepdad is transferred overseas and her mother follows him there, leaving Mitsuki to live alone with her new stepbrother, Yuya. Feeling abandoned by her mother, she stays distant from Yuya, never really getting along very well with him nor even keeping a conversation with him.

One day, Mitsuki is visited by an angel/ghost named Hiyori, who takes over her body and make sexual advances towards Yuuya. Hiyori is the lost spirit of a girl who died but can not reach the gates of heaven and so obtain eternal peace. In order to reach heaven, Hiyori puts a chastity belt on Mitsuki that fills up every time Mitsuki has sexual feelings from Yuuya, whether it be from simply going on a date with him, to kissing and even sex. For every time it fills up, Hiyori gains a step on a bridge that connects her current situation and the literal gates of heaven. If she wants to get rid of the annoying Hiyori, Mitsuki has to make some bold and sexy moves against her own brother, even if he is just a stepbrother.