Nagisa Shiota summon a Devil to his life, named, Karma, Nagisa wanted him to take his soul, but the demon refused and said he couldn't, not because he didn't wanted to, it was because he couldn't. Every demon or devil have a contract with a human to be owned or for they can own, but for Nagisa and Karma, Nagisa didn't sign a contract, and for him to sign a contract, Nagisa needed to go to a different dimension were you can sign contract, Easy right? But the only problem is that, Karma was already in a contract with one of Nagisa not-so-close-friend, Manami Okuda.