It seems some sort of monster is causing havoc in Busytown Lake. The kids catch glimpses of it and find numerous finny footprints by the shoreline. However closer investigation reveals it to be Mr Fix-It?s latest invention ? a swim fin-wheeled amphibious submarine car! When a reckless driver does some off road damage around their Busytown neighborhood, Huckle and his pals resolve to track down who's responsible. The mystery is solved when the kids discover it's not a bad driver banging into things, its Mr. Fix-it's new self-propelled shopping carts, making their way back to the supermarket.