A "Brain" is discovered in a region of Japan and an disposal mission is ordered. It ended in a huge massacre. A new elite unit is then called to deal with this threat. It's the unit "5121", consisting of young men and women. A new mysterious rookie also happens to lend a helping hand.

The mecha of Hayami and Mai does not respond and can not be found on the radar. They find themselves completely isolated from the rest of the unit and in enemy territory.

It's Christmas again and Mai finally decides to reveal her feelings for Hayami.