The year 2080. With the invasion of the Meteoras, alien beings of immense size and strength, humanity sought safety and salvation deep underground, establishing Augmented Tokyo.
This 2km radius underground city became humanity's last bastion.

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The year 2280. Chloe, member of the Anti-Meteora military organization Prometheus, defends the city in her Makhia, a massive battle robot. Though her foes are formidable, she survives with the help of her companions.

Prometheus fights to protect the city from the Meteoras and retake the surface one day. However, that is not why Chloe pulls the trigger.

No. Everything she does, she does for her fallen friend.
Her dearest friend. Coco.

The blue flame of vengeance that burns within her is never extinguished.
It sears Chloe's heart as she thinks of the choices she faced and decision she made, back in that moment.

And then, the gears of fate begin to turn at a screeching pace.

A Meteora with Coco's form attacks.
This is Chloe's reality.

[From English official website]