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.

When Zim gets annoyed by Gir's inept nature for the last time, he finally gets around to upgrading Gir's guidance technology and he wishes to test its capability. Initial tests look good so Zim wants to take Gir into the city to test and see if Gir can get them safely back to their headquarters. However, when Gir forgets to bring the new guidance chip with him, Zim and Gir must find a way to get back home, despite the fact that they have very little understanding of the way the city functions.