This game is very similar to Galaxian, except that there is now an energy meter at the bottom that decrements with time. If you do not destroy all the aliens before this meter empties, your ship is destroyed.
Aliens behave in the same manner as those in Galaxian: they wait at the top of the screen without dropping any bombs for you to shoot them, and eventually some will peel away from the formation and attack your ship. Occasionally, one of the top-row aliens (the "flagships") will attack with one or two escorts in two; shooting the "flagship" at that point will yield a varying set of points, depending on the number of escorts and whether the escorts were destroyed first or not. The joystick moves your ship left or right, the button fires its shots.
Unlike Galaxian, you can control the direction of your shots with the joystick. When one formation of aliens have been destroyed, your ship is refueled and another formation will appear, attacking your ship more aggressively and more commonly.