Synopsis
The book begins by introducing the character of Ms. Frizzle and describing her unusual teaching methods. Soon, she decides to take the class on a field trip to the waterworks, which the kids are sure will be boring, especially compared to the trips the kids in other classes go on. However, after driving through a tunnel, the bus becomes plastered with images of octopuses and everyone inside finds themselves wearing scuba diving outfits.
Once this occurs, the bus rises up into a cloud along with evaporating water. Ms. Frizzle makes all the kids get out of the bus by threatening to give them extra homework if they don't. However, the kids begin shrinking once they're outside and, once they're each the size of a raindrop, they rain down into a river, which carries them into the town's water purification system. After going through the waterworks, the pipes take the class back to the school. They come out in the girl's bathroom, where, once out of the faucet, they are instantly restored to their regular size and normal clothing.
Ms. Frizzle, however, appears to have no memory of the strange trip and the class later sees the bus outside. They wonder how it returned from the cloud and even consider that they may have imagined their whole adventure. The book ends with Ms. Frizzle informing them that they will be studying volcanoes next. The main story is then followed by two pages listing things that couldn't happen in real life.
Read more about this topic: The Magic School Bus At The Waterworks