The protagonist, Rumi, has a perfect stepbrother, Rihito, who is two years older than her. Rihito is always kind and straightforward about what he likes. He is popular with the friends around him, and someone is always after him. Although they can't hang out because she's his sister, Rihito is always kind to Rumi. One night, Rumi went to bed without drinking the cocoa that Rihito always makes for her. When she wakes up feeling short of breath, she finds Rihito there...