From across the galaxy, a terrifying alliance of alien warships enters our atmosphere. Luckily for the Earth's human leaders, they have a prison full of monsters (whose lives they don't give a damn about) that they can strong-arm, blackmail, and guilt-trip into battling for free. Inmates of both Kaijumax prisons suit up for the filthy, dangerous work of digging radioactivity lines, dropping saucer-retardant liquid, and proactively destroying small cities before the aliens can, all in a bid to lessen their sentences.