Umenosuke-kun is a girl in the boys' dormitory

Umenosuke lives in a university dorm for guys, where he spends his days doing muscle training as a hobby, amongst his valued male friends and fun-loving seniors. He should have been living a perfect life in the dorm, but one day, he suddenly turns into a girl!? What about the muscles he's built up over the years? What about the friendships he has cultivated with his best friends? How will he live in the male dormitory in the first place!?