Destroy the aliens which descend from the top of the screen. They attack in a formation of five by eleven and there are four protective guards at the bottom behind when you can hide. Movement, left or right, is controlled by joystick or keys.
You start with three lives. An extra life is obtained when the score appropriate to the skill level is achieved. If the invaders reach the bottom the game ends regardless of lives remaining. If you succeed in destroying a wave of invaders, another takes its place, but lower down this time. At intervals a mothership flies across the top and there are bonus points if you destroy it.
At the start of a session you can choose the background colour and at the beginning of each game you select from three skill levels. Current and highest score are displayed. Graphics are very good.
It's fairly easy to destroy a wave at the first level but extremely difficult at level three. I enjoyed playing at all levels. Full instructions are on the insert but the print should have been larger.