Shoot as many invaders as possible while avoiding their fire.
- Shoot invaders! Clearing a screen of invaders will take the player to the next level.
- The invaders move in a formation from side to side; once they reach the edge of a screen the formation will drop down a notch closer to the player’s laser base. The invaders move faster as their numbers are whittled down.
- The invaders start shooting, which being hit by an invader’s fire will cost the player a laser base and the game will end when there are no more bases left in reserve. If any invader reaches the very bottom of the screen, this will also end the game.
- Bunkers can be used as protection from invader fire, although they start taking damage when hit by the player's or alien fire.
- A bonus U. F. O. flies overhead a few times during a level which is worth big points.
- Use the Z or X keys to start a game.
- Use the arrow keys to move the laser base.
- Use the Z or X keys to fire.
- Use the Escape or Enter keys to pause the game.