Hiroki Kamiya is convinced that women have it easy, especially his lazy sister Yui who spends her days playing games and doing nothing. One evening, after updating his 'Observation Diary,' Hiroki receives a cryptic message detailing how to transform into a girl. Initially skeptical, he decides to try it after a heated argument with Yui. The method involves body-swapping with a stranger, allowing Hiroki to experience life from a female perspective. However, upon returning to his own body after a brief stint as a girl, he is horrified to find his sister dead and a knife in his hands. Determined to uncover the truth behind Yui's murder, Hiroki focuses on his only clue: Mikuni Mikuriya, a popular idol whose body he temporarily inhabited. With a thirst for justice, Hiroki is prepared to go to great lengths to solve the mystery and bring the killer to justice.