The Xevians have arrived to reclaim Earth: they are the descendants of the planet's original inhabitants, returning after a long absence to take back what they believe belongs to them. Their opening move was constructing enormous ground fortresses across the surface. The pilot of the Solvalou spacecraft flies alone as Earth's last defense — destroying those fortresses from the air, neutralizing the Xevian fleet, and dismantling the treacherous SOL bases before the reclamation can be completed.