Toby lives a lonely life at the seaside hotel where he was abandoned as a baby \- until he finds a pale, thin girl lying on the beach at the water's edge, with a long green tail swaying gently in the shallows. Eliza Flot is a stranded mermaid and she needs Toby's help. He takes to her like a fish to water, and when he meets her noisy parents he practically becomes part of the family. But Toby's new friendship arouses suspicion back at the hotel. Toby and the Flots must come up with a plan to rescue each other!