In a world under attack by the rampant M34 virus, the NOA soldiers stand as the best line of defense. One soldier, Yuuji, is torn between duty and his place in the world until a chance meeting with a shy girl named Ai.
Yuuji is hospitalised due to injuries from the last battle as the rest of the NOA team heads off into the battle. In the heat of the battle, Ai discovers the horror of the M34 virus as Yuuji confronts her fears and his own insecurities.
The NOA soldiers return to the home base to begin what they think is a break from the action. One by one, they begin to show symptoms of being infected, except for Yuuji.
As the virus outbreak grows out of control, Yuuji slowly recovers from his injuries from before. In the face of growing danger, Yuuji begins to realise how much Ai means to him.