Charter captain David Hope is searching for a client to round out the season. Enter Sally Moffitt, an underwater nature filmmaker determined to make a documentary to outrival all others, who blows onto the scene and rents David’ s sailboat. Before long, the duo glimpse something incredible yet frightening which has never been caught on film before. When they encounter a research vessel with outlandish yet amiable naturalist explorer William Tree at the helm, Sally keeps the details of her fascinating find a secret— not realizing that Tree knows much more than he admits. Before they know it, David and Sally are caught in the middle of an aquatic disaster— an experiment that has gone dangerously awry, as a mysterious and ominous presence lurks beneath the surface, waiting for them to make one wrong move . . .