The year is 2045 and earth radar stations have detected numerous fleets of alien ships, bound for earth. The ships have refused to identify themselves and have destroyed a cargo vessel en route to Uranus. Their intentions are presumed hostile. One last defensive position stands between the aliens and Earth. This base is situated on the moon, just to the south of the Sea of Tranquility. The lunar base is under partial computer control but the actual task of destroying the alien ships is your responsibility.