It's a vertical scroller where you defend the earth against aliens for what must be the 200th time. Has a pretty decent animated intro. The speed settings on this game are messed up on Anex86, and it doesn't seem to like the BIOS on other emulators. I suggest you play it on Anex86 and fiddle with the speed settings.
Arrow keys move your ship, X fires. Power-ups appear to activate automatically.