Walkerville is debating whether to build a mall where the swamp is now, or to build it somewhere else. With the toss of a coin, Ms. Frizzle's class finds themselves defending the swamp and Janet arguing against them. However, the class finds out that the swamp is home to several organisms, an argument Janet finds little more than a joke, with which the city agrees. That is, until a tree gets knocked over by lightning, blocking off the swamp and flooding the city. After the crisis is resolved, the city realizes that if it were to build a mall on the swamp, then that mall would be washed away.