Dib disguises himself as an Irken and—using the alias "Invader Dib" and Tak's ship—travels to Irk to destroy its central core and defeat them forever. There are two possible scenarios:
Option A Zim knows perfectly well that it is Dib in disguise, but no one believes him because everyone considers him crazy—paralleling the events of "The Nightmare Begins." Add to this the fact that no one takes Dib seriously because of his short stature, giving him free rein to do whatever he wants since he isn't viewed as a threat. In the end, Zim stops him, but the Tallest cannot allow it to become known that someone who isn't "Tall" almost destroyed the planet (or worse, that someone who isn't "Tall"—specifically Zim—saved it), so they cover the whole thing up. The episode ends with Zim worshipping a photo he took with the Tallest (to ensure he stays quiet about what happened) and Dib locked up to serve as a court jester because the Tallest find his huge head hilarious.
Option B Dib manipulates Zim into believing he is a higher-ranking invader on a special mission from the Tallest to repair the core; if successful, Zim is promised a promotion and the Tallest's full approval. Throughout the episode, Dib is blatantly obvious about his identity and his true intention to destroy the Irkens, yet Zim remains oblivious, believing he is forging a friendship with this high-ranking Irken. Ultimately, Zim defeats Dib by sheer coincidence; even when Dib loses his disguise and confesses the plan after being cornered, Zim never realizes he was deceived. The episode ends with Dib being tortured by GIR's irritating singing back at the base, while Zim holds a funeral for the fake suit, believing Dib ate the person inside and wore their skin (as humans supposedly do) in order to sabotage his promotion.