Morgan complies and destroys them all...all but one, which he hides away. With the destruction of the majority, an alien race decides the time is ripe to invade earth. They target the last robot first, much to their regret. It fights back, and ends up destroying the invading fleet. In the process though, it has suffered irreversable damage and "dies". Ironically, when it discovered, humans never realize that it saved them, but only think it was another example of the robots going rogue.