When Uehara comes to Momoi's house to give her the homework she missed, he finds her attached to the wall as her mad-scientist grandfather is about to test his machine on her. He tries to save her, but unfortunately, the girl he loves turns on him and convinces her grandfather to use Uehara as the test subject instead. In an attempt to get Uehara, the grandfather accidentally presses a lever which causes Uehara and Momoi to switch bodies.
Because he is in her body, Momoi often threatens Uehara not to do anything to her "pure" body or else she will go out in public in his body with nothing but socks on. Uehara, fearful, obliges, though there are some cases in which his curiosity gets the best of him, usually resulting in a violent nosebleed. As a girl, Uehara in Momoi's body is more well-liked by her peers, to the extent that some males develop a crush or feelings toward him/her, including Uehara's best friend Senbongi, and Shiina's older brother.
He is very adamant about returning to his normal body, though there are many factors that prevent him from doing so. (i.e. Momoi, Momoi's grandfather being lazy, Senbongi, etc.)