Aya, Youji, Ken, and Omi are the four young men who make up Weiss Kreuz (WK), an assassin group that eliminates specific threats to society. Each member possesses a distinct fighting style. Aya is skilled with a katana, Youji has mastered strangulation using strings, Ken is deadly with his metal claws, and Omi specializes in various projectiles. Operating under the command of a mysterious figure known only as "Persia", WK is sent to dismantle drug rings, terrorist factions, and similar organizations. However, as they proceed, they realize that most, if not all, of the criminal elements they have encountered are somehow linked to the Takatori family. The Takatori family is wealthy, powerful, and influential. It appears that to stop the problem at its source, WK would have to destroy one of Japan's most prominent clans.