While returning from a "battle mall" where he'd bought a new arsenal of weapons, Zim is distracted by the space slug that GIR bought, crashing the Voot Runner on a nearby planet that's nothing but a barren rock. Zim attempts to repair the damage to the Voot, but as he does, fluid leaking from the ship mixes with the planet's core, somehow creating a primal ooze that induces accelerated evolution in all it touches.
By the time Zim notices what's happened, the planet is no longer lifeless, having developed a jungle-like ecosystem. Among its inhabitants are a race of amoeba-like creatures who worship Zim, and a species of ladybug-like predators the amoebas call Blerks. The latter steal the Voot, stranding Zim on the planet. To get it back, Zim throws GIR's slug into the ooze, making it gigantic and arming it with all his recently purchased weapons, dubbing it the Battleslug 7000, which he uses to wipe out the Blerks and retrieve the Voot.
Afterwards, the amoebas repair the Voot, before begging Zim to destroy them, as they believe it would be a sign of his approval. A confused Zim agrees, arming a bomb that detonates as Zim and GIR leave, destroying the planet and everything on it.