While tidying up the Calligraphy Club room, Kiryuu Keiki stumbles upon a surprising love letter left on the desk addressed to him. Excited about the prospect of romance, he eagerly opens the envelope, only to discover a simple yet bold confession of 'I like you' accompanied by a peculiar item – a pair of the sender's panties. Armed with this unusual clue, Keiki embarks on a quest to unveil the mysterious sender among four possible suspects: the laid-back senior club president Tokihara Sayuki, the foreign library committee member Koga Yuika, the slightly aloof classmate Nanjou Mao, and lastly, his own perfectionist younger sister Kiryuu Mizuha. Determined to solve the enigma, Keiki delves into the eccentricities of these girls, determined to unravel the identity of his potential girlfriend amidst this unconventional love story.