This is a conversion of a highly acclaimed Mac title. The premise is that aliens come to wipe out the earth and only you can stop them...geez, anyway. You are a soldier based on the Lunicus moonbase, earth's last defence. The game progresses through missions in which you go back to earth, clean up a specific place of alien scum in standard fps fashion, and go back to the moon. Eventually you'll have to defend you base and finally you'll take on the alien's mothership.
The game includes a lot of cut-scenes and you can chat with your fellow soldiers aboard the lunicus between missions.