Marina DeRavine seems like any normal 17 year old. She goes to school, attends drama club, and even has a crush on the local golden boy. Totally normal right? But one day when Bay, the incredibly mysterious (and undoubtedly enticing) new guy in school, shows up with answers to Marina's hazy past, she is taken away to an underwater world of Merpeople, adventure, and most of all, trouble. As Marina discovers that the fate of the oceans lies in her hands, she and Bay must set off to uncover the ancient legend to save their two worlds. But as they dive deeper into the mystery that Marina comes to find is somehow linked to her father's mysterious death, her chronic nightmares, and the secrets her mother has been keeping from her for years, things begin to get complicated between Marina and Bay. The closer they become, the more they understand that they are dealing with some very dangerous people hidden beneath delicate waters.