In order to ensure the protection of a 14-year-old girl stuck in a succession war between Mafia clans and other powerful conglomerates, the serious inspector Hatori is forced to collaborate with the whimsical Kuroumaru Wakatsuki, a legendary assassin, known under the name of "Black Dog."

Will this poorly matched duo get along enough to pull young Mana from the clutches of the horde of killers on his heels? Thus begins an infernal chase on the trail of the most fearsome assassins in the world!