Lily (Ellie Bindman) and her family have moved to a small seaside town. Lily misses her friends, the hustle and bustle of the big city. Determined to do nothing but stay indoors for the summer, her plans are pushed to one side when her mother Claire insists she goes out to get some fresh air. A walk on the beach changes everything and magically, somehow, Lily becomes a mermaid. Lily grapples with this discovery whilst keeping it a secret from her parents and younger sister Sophie (Georgia Grace), but before too long, she discovers she has an affinity with the sea she never thought possible. Meanwhile, a land developer has plans for the town, which could have a devastating effect on the beach and the sea that Lily has grown to love.