The Irken race consists of beings that have desires of gaining universal domination. Their leaders, the Almighty Tallest Red and Purple, have just begun assigning Invaders, the elite soldiers who will capture the universe's most dangerous planets. However, when Zim, an Irken soldier who was banished from the Irken Empire, returns and demands to be assigned a task as an Invader, the Tallest decide to send Zim to a "planet that nobody has ever heard of": Earth. Zim gets assigned a cheap robot that is malfunctioning and he begins the trek towards Earth, but little does he know that a human child on Earth named Dib is already aware that he is coming, and plans to prevent Zim from completing his goal.
Spoilers ahead. Coverage follows the scope labeled below.
Complete story — includes the ending
The Irken race is having a great assigning for Operation Impending Doom II, where all Invaders are assigned a planet to conquer. Wanting to rid themselves of the irritating and arrogant Zim, the Almighty Tallest give him a faulty SIR unit (named GIR) and send him to the farthest reaches of space. When Zim arrives, he finds a planet named Earth where he sets up base and, in disguise as a human, infiltrates the local school. Unfortunately, one schoolmate, Dib, is a young paranormal investigator, who vows to capture Zim and prove that he is an alien.