Tokyo Underground Killer follows Kobayashi, a mercenary nicknamed the Shinjuku Vampire, as he works in the criminal underworld of the metropolis. The character takes on contracts to eliminate dangerous maniacs using a sword, unique skills, and powerful weapons. The key task is to stop the invasion of the brutal Flatliners group. The basis of the combat system is tied to the management of blood energy, which feeds the killer's mystical skills and powers during intense fights.