15 years ago, the newborn Ukai Nanoha and her mother were involved in a plane crash. The mother was killed and the child alone, among all of the passengers, survived. This survival was brought about thanks to the Great Demon Maou, who replaced her unbeating heart with one of his own, allowing her to live on.
Now, however, Maou has appeared once again and attempts to force her to return his heart. In the end, Nanoha goes to live in Mezon Aasu (Earth Mansion) which exists solely for the use of magical beings, and for now at least, retains possession of the Great Demon Maou's heart... but how will she be able to handle being manager of Earth Mansion, the place in which Maou himself makes his residence?