Toby lives a lonely life at the seaside hotel where he was abandoned as a baby - until he finds a 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. 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.