He was supposed to drown her. Sadie has spent her life chasing the horizon-working aboard her father's crabbing boat and laughing off myths of the deep-until Kyros boards their ship. There's a storm. Blood stains his hands. Before she can understand what's happening, Kyros throws her overboard and leaves her to die. Except Sadie doesn't drown. She isn't human. She's something forbidden. When Kyros realizes what she is, he takes her back to Tideborn-an underwater city on the brink of collapse. Only somebody with Sadie's blood can fix it. But there are monsters that lurk in the depths, the Red Bloom is near, and Kyros-the man who betrayed her-is the only one who can keep her alive. He says she's their last hope. Sadie isn't sure she wants to save his world. She might rather watch it burn.