Hiroki Kamiya believes that women have it easy. Every day his sister Yui lazes about, playing games and doing absolutely nothing. One night, after Hiroki updates his "Observation Diary," he receives a mysterious message telling him how to become a girl. He dismisses it at first as a scam; however, after an argument with his sister, he follows the instructions to experience the carefree life of a girl. The method allows a person to swap bodies with a stranger—letting someone enter your body while you enter another's. After spending two hours in a girl's body, Hiroki returns to his own but is met with a shocking sight—Yui's bloody corpse in his arms and a knife clenched in his hands. Who entered his body while he was gone, and why did they murder his sister? Hiroki's first lead is the girl whose body he entered: Mikuni Mikuriya, a famous idol. Desperate for answers, he will do whatever it takes to bring justice to his sister's murderer.