While trying to get down to under, Cliff is bitten by a Frobish, catching Monsteritis and thus turning into a monster himself. Russel goes to the Uncertain City to get the slime of a fairly large Dongo Worm, the only cure for Cliff. Bob and Auntie also want the worm for their own uses, and Gus tries to organize with Captain Darling and the Frags so they can capture the worm together and escape the city before it blinks out of existence.