Kyuu and Nanami are childhood friends and they’re both members of the occult club in school, although Nanami is more interested in it than Kyuu. On day, the club’s chairman Saeka tells them that there’s a really good second-hand shop which might have some occult items in it. The visit the shop and Nanami ends up buying a book called “Grimorium Verum” which apparently allows the reader to summon Satan. On the night of the full moon, Nanami tries to perform the ceremony to call the devil and she actually manages it. The figure that appears calls itself Mephistopheles and says that in exchange for Nanami’s soul, he will give her great power. Kyuu tries to protect Nanami and obstructs the ceremony, however, he is thrown onto the ground and Mephistopheles announces that his contract is complete and that both Nanami and Kyuu are now “contractors” but they have to fight together.

[From Liemyx's Blog]