Save the humanoids from the impending aliens in the sequel to Defender. The task is still rescuing humanoids before Landers can turn them into Mutants, while avoiding and shooting other foes, however you must now carry humanoids to safety through the Stargate of the title.
Entering this Stargate not only warps you to the nearest humanoid in jeopardy, but can also warp you ahead (while on the first 15 levels) if you enter it with 4 or more humanoids, and give you extra lives if you warp with 10 humanoids, so there is a trade-off between guaranteeing the safety of existing humanoids, and trying to advance your position
There are more enemies on screen than ever before. The Inviso button makes you invisible to enemies, but also to yourself, so you will have to follow your bullets to work out where on the screen you are.