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.
Keiki finds a love letter on his 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 half-foreign library volunteer with an enigmatic smile. Mao, his classmate with a sharp tongue hiding softer feelings. And most awkwardly, his own younger sister Mizuha, who maintains an obsessively clean facade. 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. The mystery deepens with every interaction, making him wonder if the real pervert here might be him for enjoying the hunt so much. Torn between genuine affection and fascination with the bizarre, Keiki navigates a world where normalcy is just a comfortable lie everyone agrees to maintain. Romance has never felt so weird.