In the city-state Risvaletta, while people go about their tranquil everyday lives, crimes and illegal drug trafficking run rampant. Above all, a dangerous, highly-fatal drug named "Anthem" is casting a dark shadow on the streets. The "Seven-O" special criminal investigation unit whose specialty is to supervise the Anthem investigation, institutes a "Double Decker System" policy of forming groups of two investigators, and all members are given nicknames.
One team consists of the veteran investigator Doug Billingham accompanied by the rookie Kirill Vrubel. This unconventional duo combines an aloof veteran detective who is hard to get a read on, and an inexperienced detective who is a little too eager.