The alien invaders are coming to attack Earth, try and destroy them as they break formation and attack. For 1 or 2 players.
So of course after Space Invaders one of the games that I played at the arcades was Galaxian. This was a bit more of a stretch for a Basic program as a larger number of enemies needed to be moved, as well as the attack patterns of the diving aliens.
Only one alien dives at a time, and there are only two types, but not a bad attempt at making a more complex game by my early programming self.
See the video here.