Adapted from the Williams' coin-op, Defender places you in the middle of an alien invasion of Earth. You are the pilot of the Universal Space Ship Defender, and your mission is, of course, to defend the planet Earth and save humans from being abducted by the invaders. Keep an eye on the scanner for approaching enemies. When the battle begins, you have unlimited laser power and three smart bombs to use in the attack. For a quick escape you can also make use of a hyperspace time warp.

Each wave ends when all enemies have been destroyed. Aliens appear in five forms: bombers, baiters, landers, pods, and swarmers. If a lander captures a human and takes them off of the screen, the human will return as a mutant and wreak havoc. When five humans have been abducted, the mutants successfully takeover the Earth, but gameplay continues. Defender offers 10 variations for a one-player game and ten more for two-player games. A game ends when a player loses all of their lives.