Kiryuu Keiki came across a love letter with his name on it, accompanied by a pair of panties from an unknown sender while cleaning up the Calligraphy Club room. Keiki was thrilled at the possibility of finally having a girlfriend and eagerly tore open the envelope. However, he found a simple message inside that read "I like you." Keiki soon realized that one of the four girls who had helped clean the room that day - Tokihara Sayuki, Koga Yuika, Nanjou Mao, or his own younger sister Kiryuu Mizuha - must have been the sender. To discover the identity of the mysterious sender, Keiki decided to flirt with each girl and see which one had sent him the unorthodox love letter and panties.