After a junior detective dies in the line of duty during an active investigation, Detective Masaki Sara of the Tokyo Metropolitan Police First Investigation Division is transferred to Section 1 of the Personnel Division, the internal affairs unit that investigates officers suspected of misconduct. Still traumatized by his past, Masaki is forced to carry out his duties as a lonely outcast as his colleagues shun him as a snitch.

One day, a message arrives. It's a tipoff that Saiko Minaguchi, Masaki's former colleague in the First Investigation Division, is leaking confidential information. Although Masaki is reluctant to investigate someone he used to work with, he obeys his boss and starts looking into Saiko. He soon discovers a link to an unresolved murder and some buried facts about the death of his junior detective Saito. What shocking truth awaits Masaki beyond his suspicions and inner conflict?