The object of Defender is to score as many points as possible by defending the earth and the humans from wave after wave of evil invaders from outer space. You are in command of the Defender ship with its laser cannon and smart bombs. Although there are many different aliens, the primary attacker is a Lander whose mission is to come to the earth and steal all the humans. If the Lander is successful in abducting a human and taking him to the top of the screen, they are both transformed into a mutant which is faster than a Lander. Watch out! Mutants are both tricky and fast. If all the humans are stolen, the earth will be destroyed and you will find yourself in outer space fighting fleet after fleet of mutants.