Sometime in the future, somewhere in Europe, a city features multiple underground levels that are secret, dangerous, and hold answers to the past. Mihai is a retired assassin who has returned home to settle a vendetta and begin a new life as a restaurant owner. Haine was subjected to genetic experimentation as a child, which resulted in him possessing dog-like attributes. Badou is his clumsy, chain-smoking partner who becomes deranged when denied cigarettes. Naoto was orphaned as a child, suffers from amnesia, and was trained to wield a sword. Their pasts are somehow connected to the secret underground and the organization that controls it.