Miriam, Derek, Ian, and Jenny are overachieving high school students doing everything by the book. Straight A's, sports, yearbook, band, and - when coursework allows - planning and executing elaborate murders.Sisters Lisa (Mandy Moore) and Kate (Claire Holt) are on vacation in Mexico after Lisa's boyfriend recently broke up with her. They decide to go watch sharks from a diving cage with two local men. At the docks, Lisa is wary of the boat and its owner, Captain Taylor (Matthew Modine). Kate has had one lesson in diving, but Lisa has never dived. They lie to Taylor and tell him that they are both experienced divers. Unbeknownst to everyone, the winch supporting the cage is unsecured.

Taylor sends Lisa and Kate down, with masks that allow them to talk to him on the surface and to each other. They are soon surrounded by great white sharks, but the winch boom breaks free and the cage sinks to the ocean floor 47 metres down and out of communication range with the boat.