Zim gatecrashed the Great Assigning and demanded to be assigned a planet. Desperate to get rid of Zim, the Tallest told him to go to a "planet so mysterious, no one has even heard of it'. Then they showed him a sticky note with the word "Planet?" and a question mark. The unbeknown planet to the Irkens happened to be Earth, with Dib picking up the transmission.
In the episode, the Almighty Tallest are preparing to promote graduates to Invader status, in which Zim interrupts. Due to them wanting to both get rid of Zim and start Operation Impending Doom II (as shown in a flashback to the first Impending Doom, Zim ruined it by destroying his own planet, unwittingly) they launch Zim to "Planet?", or Earth.
Tak makes a reference to the Assigning in her debut episode, stating that she should have been there.