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 must venture into the ocean's darkest depths to help find mermaid Pearl's missing shell. They know the Dark Queen's sea snakes are on the prowl... but nothing can prepare them for the discovery that Pearl's shell is inside a huge underwater volcano - a volcano that could erupt at any time!