Shinjuku turns into a dangerous place as a series of incidents happen every month. The group called Adonis is behind these events referred to X-Day cases.
The protagonist is Hoshino Ichika, a newbie policewoman who does what she can for her job. While in the middle of doing her work, someone attacks her and she wakes up with a collar. Yanagi Aiji, Sasazuka Takeru, and Enomoto Mineo, former police officers who investigate the X-Day cases, found her. They all learned that to remove the collar, they must discover the truth behind the X-Day events before the big day which will happen at the start of the new year. Hoshino has to work with the guys to save her life and eventually, Shinjuku, without telling the police or else she dies.