Toby can't remember a time when he didn't live at the seaside hotel. Left there as a baby, he's grown up without many friends. One windy day, he's sent down to the beach to look for Cook's lost laundry. It's the noise that attracts him first, like teeth cracking on seashells. Then he sees her, a pale, thin girl, lying at the water's edge with her long green tail swaying gently in the shallows.

Eliza Flot is a stranded mermaid and Toby takes to her like a fish to water. When he meets her extraordinary parents, he practically becomes part of the family! But Toby's new friendship arouses suspicion back at the hotel.

Can Toby and the Flots come up with a plan that will change their lives forever?