It's a hot summer day in Dimmsdale, so Mr. and Mrs. Turner decide to go see a movie, leaving Timmy in Vicky's "care". Vicky orders Timmy to go up to his room, where he complains about summer being so boring and hot to Cosmo and Wanda. To help their godchild cool off, Cosmo and Wanda turn Timmy's closet into an ice rink. Timmy goes to lock up his bedroom door so that Vicky can't intrude on anything, and then plays a game of hockey with Wanda as the goalie and Cosmo as the hockey puck.
An hour later, Wanda catches a cold from all the ice, and Cosmo gets a headache from having been knocked around by Timmy's hockey stick so much, to the point that he continually hears bells ringing. Timmy suddenly notices Vicky unlocking the locks on his bedroom door and wishes that she never found his keys, but Cosmo and Wanda mishear that as Timmy saying he wants monkeys, so they poof up four monkeys in his room. Vicky barges in and yells at Timmy, at which point the monkeys chase her through the house and out the front door. Timmy goes after the monkeys, but not before Wanda and Cosmo mishear his statements again and think he has wished for a horse.
Timmy manages to gather three of the monkeys using the horse, but after Cosmo and Wanda poof them away, Timmy realizes there is still one left, chasing Vicky up a tree. Timmy wishes for Cosmo and Wanda to make the monkey go, but they instead make it grow to an enormous size. The monkey takes Vicky in his hand and climbs onto the roof of the Turners' house, which Cosmo then makes grow to skyscraper size when he mishears Timmy saying "I wish I'd kept my mouth closed." Timmy wishes for a helicopter, making a whirling gesture so that Cosmo and Wanda can finally understand him, and he then flies to the top of the roof to save Vicky. But she refuses his help because he doesn't have a drivers' license, and then calls the monkey a "smelly ape", which hurts his feelings. The heartbroken monkey grabs onto Timmy's helicopter to get away from Vicky. After Timmy lands the helicopter, Cosmo returns the house to its normal size and Vicky climbs down from it just as Timmy's parents return home.
Later that evening, after Cosmo and Wanda have recovered from their illnesses, Timmy heads up to his room, but Wanda warns him not to do so too late. A huge pile of snow that has gathered up in Timmy's room falls right on him, causing him to catch a cold.