Broken bicycles, battered mattresses, and used diapers (yuck!). Penny and her friends can't believe their eyes. They find all these things strewn across the grass behind the park. Who would do such a thing? And while they go to collect them, they learn from the park manager that someone often dumps trash there on Wednesdays. Penny and her friends become suspicious: there seems to be a system here, and therefore a culprit. But the search for clues leads nowhere. Only when the four detectives resort to the toughest (=most embarrassing!) measures do they get on the trail of the shameless criminals...