A pair of panties arrives folded inside a love letter left for Keiki Kiryuu in his school's calligraphy room, kicking off this light novel's search for the girl responsible. Keiki narrows the suspects down to four: Sayuki Tokihara, Yuika Koga, his adoptive sister Mizuha Kiryuu, and newcomer Mao Nanjou, only to discover each one's affection comes wrapped in an increasingly unusual fetish. This is the original novel the manga and anime later adapted.