One night, Shihou Souma and his childhood friend Katsuragi Marina are walking in a park after school, when human-like monsters suddenly appear. These monsters, seeming to come from another world, start attacking and raping people, to absorb magical power from them. In this chaotic situation, Amakubo Yuri, Souma's school infirmary doctor, shows up and givs ExS-Device to Marina. Using the device Marina transforms into ExS-TIA and fights the monsters.

[Translated from official site]