The world Denji was born in was not at all happy for him: his father committed suicide, leaving him with a mountain of debts to the Yakuza, and there are demons everywhere, just looking to drag someone into a back alley and eat them. He has a tendency to kill demons in order to free himself from debts, but he must also kill them in order to have a normal life. But with one of them, Denji manages to become friends and make a contract. After his father's death, Denji meets a small, wounded demon named Pochita. After giving him his blood, he healed him and made him a promise that each of them would help the other survive in this nasty world.