The class 3A trip to the Natural History Museum is turning out to be a disaster: Many of Katie's excited classmates are acting up, and they turn on Katie and call her a goodie-goodie when she won't join in! Even worse, their tour guide, Mr. Weir, is a big meanie! Still, when Katie turns into Mr. Weir and causes a big disaster of her own, it's up to her to make things right--and save Mr. Weir's job!