Yusa wakes up with no memory of the past few months and finds himself next to the body of Kaoru, a senior detective with a gun in his hand. He has no idea who killed Kaoru, but the murder leads to the investigation of an unsolved robbery case.
Yusa leaves the cabin, erasing all evidence that would surely arouse suspicion that he was involved in Kaoru's death. Yusa begins to take care of Kaoru's only son, Katsuki, not knowing that this encounter is the beginning of a great change to Yusa's destiny.
Yusa is suspected of Kaoru's murder, and the police are believed to be involved in the robbery case that Kaoru was following just before her death. In the meantime, evidence found in Yusa's place links him to the robbery.
Despite the seeming conclusion that Honjo was responsible for both cases, it seems obvious that someone wants to bring an early end to the cases. This makes Yusa and his team pursue their investigation even further.
After regaining all his memories, Yusa heads to the cabin where it all started. He uncovers the truth behind Kaoru's murder, the 1 billion yen in Nippon Noir funds, the man in the gas mask, and why his memory was erased. Everything is finally revealed.