Your job is to defend the world's remaining population from an alien invasion. Patrol the landscape and blast anything and everything out of the sky, while taking care to keep the humanoids alive and on the ground.
Your main enemy is the Lander. Landers appear in packs, scouring the ground for humanoids they can kidnap, whisk off into space, and turn into Mutants. As much as possible, you must destroy the Landers before they can grab any humanoids. Humanoids cry out when a Lander succeeds in capturing one. When you hear that cry, you must find and destroy the kidnapping Lander before it can disappear up into space, then grab the humanoid yourself before it falls back to the ground. Humanoids can survive short falls, but long falls are deadly, and so are your shots, so aim carefully when making a rescue!
Humanoids pulled into space are turned into evil Mutants. When that happens, the only thing left to do is destroy them before they destroy you, and then protect the humanoids still left on the ground. If all humanoids are turned into Mutants, the entire landscape is destroyed, leaving you in space and battling for your life against swarms of Mutants! Every five levels you are assigned a new group of humanoids to protect, so if you have lost the current group, you must face armies of Mutants until after the next fifth level.
Several other alien types help defend the Landers in their quest. Bombers wander up and down the sky, leaving floating mines in their wake. Pods seem harmless enough as they float over the countryside, but they unleash small, pesky Swarmers when shot. And if you if dawdle for too long, Baiters appear, slicing through the air and homing in on your ship to distract you, or worse!
Your laser cannon is your primary weapon against the aliens, but you also have Smart Bombs and Hyperspace at your disposal. A Smart Bomb will instantly wipe out every alien visible on screen. Humanoids will survive, even ones in the air, but then they will still have to contend with gravity. Smart Bombs are great for getting out of tight jams or rescuing several kidnapped humans at once, but you only have a limited supply, so use them carefully. A new Smart Bomb is granted whenever you score enough points to earn an extra life. Hyperspace lets you jump instantly to the opposite side of the landscape. Hyperspace is great for finding distressed humanoids more quickly, and for escaping alien ambushes, but use it sparingly, because survival through a Hyperspace jump is not guaranteed!
Completing a level means killing all Landers, Bombers, Pods and Swarmers, along with any Mutants created during the level. Baiters can be ignored, though of course they will make it hard to do so. Once all aliens in a level are destroyed, you will receive a bonus for any humanoids still breathing. Then it is off to the next level and a new wave of attack!