When the Irken Empire decides to execute Operation: Impending Doom II, the leaders, The Tallest, assign a planet to various soldiers for each of them to conquer. But an unexpected visitor arrives, Zim. Zim has been banished from Irk because of the "little incident" from Operation: Impending Doom I. He destroyed his own planet. He has now returned to be assigned to a new planet. The Tallest assign him a planet that "nobody has ever heard of." They do not know that a planet exists in this location. When Zim sets off with a cheap and malfunctioning robot aid, about well, a long time later, they reach the planet, Earth. But one little human child outcast is already expecting his arrival, and is planning to stop this horrible invasion.
This episode is the first of INVADER ZIM. There would be many, many other good ones in the future but I loved this one just the same.