A series of violent murders have been taking place in the Shinjuku district since April 1, 20XX. Based on the fact that a Roman numeral is usually left at the crime scenes along with a unique coin that is difficult to duplicate, the police believe these crimes are an organized act of terrorism. After every killing, a video statement that is typically demeaning to the police and counts down the months until their final “X-Day” is released.
The government issued a city-wide quarantine of Shinjuku on September 1st. They barricaded the entire district with thick walls and wires in an attempt to prevent the atrocities from spreading further.
Along with the issuance of the Shinjuku Quarantine Act, the Swords and Firearms Law was repealed within the borders of Shinjuku. A unique serial number is assigned to each gun that was distributed to Shinjuku residents, and specific rules regarding the usage of these guns are issued.
The collar around the protagonist’s neck has a built-in poison delivery system that can be triggered remotely, which puts her life in constant danger. Realizing that this collar is key to uncovering Adonis, the protagonist agrees to team up with Aiji Yanagi’s investigation.
A group of people led by Aiji Yanagi who investigate the X-Day incident on their own. They call themselves a Detective Agency simply because of the placard left on the rundown building they’re operating out of, and each of the investigators at the “Agency” operates on their own.
This is the department of the police that the protagonist works for, also known as SRCPO for short. This department was created in order to deal with the public calling in to report activity related to X-Day. This department also fields complaints from the residents and conducts a routine patrols.
The terrorist organization behind the X-Day Incidents. They refer to themselves as such when making announcements on the video-sharing sites and it declare their purpose to be to restore the country, starting with Shinjuku. Its size and the location of their headquarters are unknown. The group responsible for the attempted assassination of the prime minister 2 years ago used the same name, so it is believed that the former disbanded group has reformed.
A unique coin is always left at each crime scene of the X-Day incidents, which has a cat and anemone flowers motif with the word “Adonis” engraved upon it.
A Roman numeral that is always left at the crime scene of the X-Day incidents. The Roman numeral IX was left in April and VIII in May, so the going theory is that something might happen next January when the countdown hits zero. Due to the difference in methods as well as the state of the crime scene in each crime scene, the police believe that each killing that is part of the countdown was performed by a different killer.