It begins with the Cat in the Hat's daughter doing one of her favourite after-dinner activities: sitting in her room thinking up fuzzy creatures with her "thinker-upper". When she gets bored, she uses her thinker-upper to "thunk" up a creature called a Glunk that is dirty. The Glunk then gets on the phone to his mother and starts discussing an obscure stew recipe but racks up a phone bill of $300 and requests that the young cat pay. When she tries to use her 'un-thinker" to "un-thunk" him, he says it's impossible. Eventually, she tries unsuccessfully to un-thunk him but cannot, that is, until her brother finds her and they un-thunk him together. The brother then gives her a talking-to and she promises to only think up the fuzzy creatures.