Twelve-year-old Macy is an amateur treasure hunter. She finds all sorts of lost things along the shores of Buffalo Pound Lake. When her mom announces she's leaving her quiet park ranger job at the lake to be a police officer in the city. Macy decides she needs to find a treasure big enough to put Buffalo Pound on the map so her family can stay put. But instead of treasure, Macy finds a mermaid! She learns that the frightened creature was kidnapped from the West Coast and brought to the Prairies by a monster known only as "The Beast." Macy is determined to help her get home.