After her mother remarries, Mitsuki Kanzaki ends up living alone with her new stepbrother Yuya when their parents move overseas. Her life turns strange when she is possessed by Hiyori Kotobuki, the spirit of a girl who must make Mitsuki fall for Yuya to reach Heaven's gate. To make progress, Hiyori locks a belt on her with a gauge of affection that has to keep filling, or both of them are lost.