Trained fighters known as Bastioners battle against invaders from another world using a mysterious power called Aegis. Aegis can grant people and objects tremendous strength, but people are unable to use it on themselves and must therefore always fight in pairs to give each other power.

Yorino Aratsugu and Yoko Kaminoi are two young girls training to become Bastioners. Yorino used to protect the timid Yoko from bullies, but now Yoko has surpassed her at everything other than combat skills. As the two of them become full-fledged Bastioners, Yorino must fight to keep herself from becoming fully eclipsed by Yoko's talents, while also struggling to understand the more complicated feelings she has for her childhood friend.