The class has gone to the beach, but Ralphie insists on keeping the class in one spot, which the rest of the class think is the most rotten spot on the beach, being crowded with other people. Ralphie, however, insists that this is where they want to be. Ms. Frizzle hears that "Uncle Shelby" needs a special spot saved. Eager to get away from Ralphie's spot, the rest of the class agrees. However, they find the inhabitant of this spot is not a human; it is a mussel. They decide the middle tide zone is too crowded so they decide to find a spot. However, they find worse problems in the low tide zone and the high tide zone, so they eventually come back. When they get back to Ralphie, they try to apply what they have learned to convince him to change spots, but they discover that he did have his reasons for picking the spot—the ice cream truck stops there!