Penelope Rex has one fear—Walter, the class goldfish. So when Mrs. Noodleman tells the kids that they'll be taking turns bringing Walter home, Penelope gets carried away by her anxious imagination. The weekend has a rocky start—from Walter ominously licking his lips to a sleepless night where the fish's bowl appears to move closer and closer to Penelope's bed. But after a mix-up with a donation box results in a visit to the local aquarium, Penelope realizes she might just need to confront her fear of Walter—in order to save him.