Harriet loves costumes. She wears them to the dentist, to the supermarket, and most importantly, to her super-special dress-up birthday party. Her dads have decorated everything for the party and Harriet has her most favorite costume all picked out for the big day. There’s just one thing missing—party hats!

But when Harriet dons her special penguin errand-running costume and sets out to find the perfect ones, she finds something else instead—real penguins! Harriet gets carried away with the flock. She may look like a penguin, but she’s not so sure she belongs in the arctic. Can Harriet manage her way back to her dads (and the party hats!) in time for her special day?