Being best friends with a unicorn is truly incredible, as 10-year-old Phoebe Howell knows from experience. But what would it be like to actually trade places with a magical unicorn for a day? As it turns out, it's much harder than it looks! Phoebe's botched muffin spell turns the sky plaid. And things aren't any easier for Marigold Heavenly Nostrils, who has no idea how to hold a pencil. The friends also meet a three-eyed flying creature named Glorpie and encounter an enigmatic ghost.