Phoebe “Phibs” Ray is never more at home than when she’s underwater. Six months ago, she and her four closest friends found social media fame after discovering a handful of ancient gold coins in the Florida Keys. Now, their final summer together after high school, they’re taking one last trip to a distant Australian island to do what they love most – scuba dive. While diving a local reef, Phibs discovers a spectacular underwater sea cave, rumored to be a lost cave with a buried treasure. But when Phibs and her best friend Gabe surface from the cave, they notice that they're undergoing strange changes. Oozing gashes that don’t heal. Haunting whispers in their heads... Something has latched onto them, lurking beneath their skin, transforming them from the inside out. When treasure hunters arrive, desperate to find the location of the cave and hold Phibs’ group for ransom, she’ll do anything to keep her friends safe. In the process she learns that, of all the dreadful creatures of the sea, she might be the most terrifying of them all.