Seven-year-old Lily loves feeding the ducks at her park pond. But one day, the pond is covered in plastic cups, bags, and straws. The ducks look sad, and the fish are hiding. Lily meets a little turtle named Tuck who has a bottle cap stuck on his shell. Tuck tells her that a "Litter Monster" is making a mess-but the monster is just people being careless. Lily gets her friends together for a pond cleanup challenge: whoever collects the most trash wins a homemade medal. Along the way, they learn that recycling, reusing, and saying "no thank you" to extra plastic can bring the pond back to life.