Sister Game is a brother-sister romantic comedy that follows the daily life of Gorō Hōzuki, a high school boy with an older sister named Haru who is unrelated by blood. Gorō is mortified when the self-centered and lustful Haru plays pranks on him.

A friend of the detective has been is murdered. During the investigation, he comes across a source which tells him that his friend was murdered for involvement with the dark side of the political world. On top of that, a woman approaches the detective with a request.