Fiona isn't crazy about spending summer vacation with an eccentric aunt and uncle she's barely met, but at least her brother and sister are stuck in Florida with her. During a spider hunt on her first night there, she's told they are in a cryptozoological vortex complete with sprites, Bigfoot, and a hornless unicorn. Life becomes stranger when her younger brother, Ian, disappears and is believed to have been kidnapped by a swamp creature. Before she has a chance to help look for him, Fiona is stolen by a kelpie and taken underwater to the world of the mermaids. Kept as a playmate for the king's youngest daughter, she transforms into a mermaid herself, just before she's told she will be sacrificed to the giant sea creature, Lusca. Will Fiona survive the transformation, or more importantly, if she does can she battle Lusca and win?