Adventure always seems to find Emily Windsnap! So when she discovers a bangle with a unique crystal under Rainbow Rocks, she and her friends are soon unraveling a whole new mystery. After learning that the crystal originally came from a familiar place—Halflight Castle!—Shona and Emily reunite there with Aaron . . . and none other than Neptune, king of the oceans. The bangle leads the mermaids to a cave full of crystals, and, with Mandy's help, they discover the truth about the jewel’s origin—and what’s at stake if they don’t make things right. Unfortunately, the only solution involves getting the sea and sky’s most powerful (and notoriously selfish) rulers to put their own interests aside for the greater good. Will the quartet find a way to convince the imperious Neptune and the Viking god Thor to come together to prevent disaster?