You've been living in an underground civilization for your whole life, and generations before. Suddenly, a Gunmen (mech) falls from the surface. There's a boy, a man, and a girl from the surface with you. The boy has a mechsuit, though it's much smaller than the one you 4 are currently fighting. Eventually, you finish fighting and get to the surface world for the first time in your life. Apparently, there's much more Gunmen lurking everywhere.

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