Arnold and Wanda are due to give a report on the town waterworks. But Ms. Frizzle thinks it's field DRIP time! She turns the bus and class into water drops and the kids evaporate, condense, become rain and rush by river into the ocean. After several trips through the water cycle, they're ready to turn back into regular kids. But the magic key that will get them out of the cycle is locked in the school bathroom! Trying to work their watery way into the bathroom, the kids go through the town waterworks and see how water is purified. Can they get to school through bathroom pipes? Or are they stuck in the water cycle forever?