The class gets ready for a nighttime parent-teacher conference when Ms. Frizzle takes the parents away in the bus. They follow in another vehicle, and see that Ms. Frizzle has taken them to... a Translyvanian castle? Ralphie is obsessed with vampires during this point, saying that Ms. Frizzle's a rogue vampire! The other kids disagree, and think that's he's gone off the deep end. While Ralphie is trying to do something to stop Ms. Frizzle... he turns the bus and all of them into bats. The students then learn the true nature of bats and almost get eaten by an owl.
By using echolocation, the kids watch as Ms. Frizzle isn't really a vampire, she's just showing the parents bats in the castle, since their kids are studying them right now. But even Keesha admits that it was quite suspicious. The kids, as bats, locate the bat-bus and transform back just in time.