For reasons undisclosed, the Earth is under attack by missles (sic) from another planet. As the unexplainably-hostile aliens rain down devastation from the sky, it's your job to stop the warheads from striking the surface. Enemy projectiles appear at the top of the screen, and weave a somewhat random path towards the bottom. There, you must intercept them with your ABMs, which can only be moved left or right (somewhat like a Pong paddle, but one step at a time). 20 "missles" are fired on each attack wave, and you must stop at least 10 to qualify. The going gets tougher as you advance, and later in the game, a faster type of rocket makes its appearance -- worth double the points when destroyed.