Izzy and Willow are being taken for a walk along the river by Emily’s boisterous dog, Billy, when he decides to chase after a water-rat. Both animals land in the river, which is so choked with rubbish that Billy can’t get out. His paw is stuck in an old bike wheel and the girls must work together to free him. The adventure gives Izzy a great idea for their next project, though, and it’s a very very muddy one…