The Fuuka Academy is a unified school (having students from primary school and high school). In the Fuuka Academy, a rumor that a mysterious monster appeared has spread, and that the only match for the monsters were girls called “HiME.” They have the abilities of HiME (Highly-advanced Materializing Equipment).
Mai transferred to the academy, not knowing anything of this rumor. However, she suddenly discovers she has the power of a HiME. After facing a few problems, she adapts to her new life as a HiME.
Descriptions taken from AniDB.net