The title is a genuine question, not a punchline, and the show commits to answering it by actually making you care which girl sent the panties. Every suspect has a real quirk that's specific enough to be funny without drowning the actual mystery, and the mystery itself moves fast enough that you're never waiting around for filler. It knows exactly what it is: a harem that earns its chaos by being honest about the pervert angle instead of pretending the fanservice is incidental.

After cleaning the calligraphy club room, Keiki finds a love letter meant for him on the club's desk, but it comes with an unusual attachment: a pair of panties. Thrilled at the prospect of romance, he becomes obsessed with identifying the sender among four suspects who were in the calligraphy club room that day. There's Sayuki, the lazy club president. Yuika, a library committee member with foreign roots. Mao, his classmate with a sharp tongue hiding softer feelings. And most awkwardly, his own younger sister Mizuha, a neat freak. What starts as a detective story spirals into comedy chaos as Keiki investigates each girl, only to uncover embarrassing secrets and questionable interests lurking beneath their innocent appearances. Romance has never felt so weird.