River Heights is hosting a duck derby to raise funds to install new benches by the pond in the park. Nancy, Bess, and George want to help, so they ask Mayor Strong if they can number the rubber duckies that will be used in the race. It's a big responsibility, but they know they can do it.

Then the box holding the duckies goes missing from Nancy's doorstep! The girls need to find the duck-napper fast, but there's a flock of suspects. Where do they even start without even a squeak of a clue?

Will the Clue Crew figure out who flew off with their rubber duckies before the Mayor finds out they're gone?