Sixteen-year-old Lina Kirk sacrifices herself to her sinking island's Witch Queen, Eva, to save her brother and her secret crush, but when she and Eva fall in love, everyone is in danger. Every year on St. Walpurga's Eve, Caldella's Witch Queen lures a boy back to her palace. An innocent life to be sacrificed on the full moon to keep the island city from sinking. Lina Kirk is convinced her brother is going to be taken this year. To save him, she enlists the help of Thomas Lin, the boy she secretly loves, and the only person to ever escape from the palace. But they draw the queen's attention, and Thomas is chosen as the sacrifice. When Lina offers herself to the queen in exchange for Thomas's freedom, the two girls await the full moon together. But Lina is not at all what Eva expected, and the queen is nothing like Lina envisioned. Against their will, they find themselves falling for each other. As water floods Caldella's streets and the dark tide demands its sacrifice, they must choose who to save: themselves, each other, or the island city relying on them both.