You must destroy the number invaders before they over-run your base. It's a finger-tapping frenzy!
To play the game what you do is use [AIM] button to change your BASE to match any number of the INVADER, that is advancing towards you. Once you change your Base to that Invader number you press the [FIRE] button to destroy the Invader.
If you destroy 2 or more Invaders in a row that add up to 10, 20, 30 etc then you get a 'n', or Mother Ship, advancing towards you which you can then destroy for more points.
The closer the the right hand side of the screen the Invaders are when you destroy them the more points you get. You have 3 lives which are displayed next to you by 3 lines next to your Base. The game has 9 levels and each level the Invaders advance faster towards you. Once you get past the 9th level it goes back to level 1.