Nine-year-old Molly Brown lives and breathes the river. Since her mother died, it's been her, her dad Paul, and Cappy, her ailing grandfather. With Cappy's medical bills, Paul's only choice is to sell everything and move. But Molly can't leave her friends, and especially the river. She has to make a plan! Years ago, her mother used to tell tales of a lost treasure hidden on the banks of the river—but was it just a legend? It's up to Molly and her friends, the "lil river rats," to journey into the unknown, find the treasure, and stop Molly from moving.