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!