Fun-loving, twenty something, sisters Kate (Mandy Moore) and Lisa (Claire Holt) are ready for the holiday of a lifetime in Mexico. When the opportunity to go cage diving to view Great White sharks presents itself, thrill-seeking Kate jumps at the chance while Lisa takes some convincing. After some not so gentle persuading, Lisa finally agrees and the girls soon find themselves two hours off the coast of Huatulco and about to come face-to-face with nature's fiercest predator.
But what should have been the trip to end all trips soon becomes a living nightmare when the cage breaks free from the boat and plummets to the ocean floor. Kate and Lisa find themselves trapped deep underwater with less than an hour of oxygen left in their tanks. They must somehow work out how to get back to the safety of the boat above through 47 metres of shark-infested waters.