Toby lives a lonely life in his pyjamas at the seaside hotel where he was abandoned as a baby - until the day he's sent down to the beach to look for Cook's lost laundry. Instead, he finds a pale, thin girl, lying at the water's edge with a long green tail swaying gently in the shallows. Eliza Flot is a stranded mermaid and she needs his help. Toby 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. If only Toby and the Flots can come up with a plan to rescue each other.