Far out in the ocean live the Shell-Keeper mermaids. While the rest of the world sleeps, one little girl joins them for some magical mermaid adventures. Molly's thrilled when she and her mermaid friends find another piece of the magical conch shell...but unfortunately a grumpy sea urchin has made his home on it, and won't move! He'll only swap it for the pirate treasure that lies in a nearby shipwreck...but what other dangers are lurking there?