During WW2 a strange new alien, the Genjuu (幻獣), appeared who's sole purpose is to kill off Humanity. Whenever they appear the Genjuu give off a strange odor that is acid to a human and killed off a lot of the warriors sent to fight it. Flash foward 50 years. Hayami Atsushi is a pilot of the 5121. The number of adults killed off in the war has forced the government to form the High School Defense force, where children of a certain age and skill are fostered in dorms to fight off the aliens. They use new mecha (HWTs) and special anti alien suits made by the Shibamura group. One day they get a new transfer student. A hot shot mecha pilot who seems to be made out of ice and is a pro at killing the Genjuu. She introduces herself as Shibamura Mai.
Who is this Mai? Why did she come to the 5121? Why does she hate the Genjuu so much.
Source: Forbin at MIKOMI