When Arnold and Wanda are told to do a report on the town’s waterworks, Ms. Frizzle thinks that the children would be better off learning about water on a personal level. Once the children board the magic school bus, it turns into a water droplet and the children find themselves being carried through the water cycle as they get evaporated and ultimately thrown back down to Earth. However, they discover that they won’t be able to turn back to normal if they don’t manage to obtain an item that is locked in the school’s bathroom.