From MangaHelpers:
Humanity has found the perfect energy source that gives the wielder near god-like powers. This energy source has been named "Demon". The rare individuals that have the capabilities to wield this power are known as "Mediums". To become one of the Mediums, Taichi has transferred to the Meissner School. On the first day of transferring into the school, he literally runs into an energetic girl named Nui. She is a Medium and attends the Meissner School. Taichi will require her help to become a Medium himself and survive against hostile Mediums.