Thea Stilton and the Thea Sisters are needed to help another magical land. The music of the sea has disappeared from the depths of the enchanted Land of the Ocean! If it doesn't return soon, the kingdom will perish. The mouselets and their friend Will Mystery travel to the underwater realm to search for the magical music. On their way, they swim through dangerous waters, reveal an ancient mystery, and encounter many hidden, fantastical creatures—some friendlier than others. It's an aquatic adventure!