In 1976, Hasbro decided to change the game for it's G.I. Joe line, hoping to improve sales. So not only did Joe get a Superheroic sidekick in Bulletman but he also got his first bonafied enemy. Before then, Joe always seemed to fight a faceless foe or an angry animal. I guess you could count the Soliders of the World but I'm not.
So after 13 years, what did Hasbro make as G.I. Joe's new foe? A caveman from outer space of course!

The intruders are a race of muscle bound savages,who have come to earth in a meteor and plan to kick a little ass. Not exactly a grand scale invasion, did I mention they're unarmed?
