Sara Masaki is a detective of Investigation Team 1 at the Tokyo Metropolitan Police Department. His colleague, Detective Saito Kota, dies during an investigation of a murder case. Afterward, Masaki is transferred to the Personnel Department 1 of the Inspection Division. He still suffers from the death of his colleague. The other workers around Masaki are hostile to him. One day, a letter reaches the personnel department. The letter claims that Detective Minaguchi Nako leaks secret information. Detective Nako used to work with Masaki Investigation Team 1. Masaki begins to investigate the claim in the letter. His investigation leads him to the unsolved case of the death of his colleague.