Lisa (Mandy Moore) and Kate (Claire Holt) are sisters on holiday in Mexico. Lisa’s boyfriend has broken up with her because “she made the relationship boring”. Kate attempts to cheer Lisa up by taking her out drinking and dancing at 1:00 am. The sisters meet a pair of local men at a bar and the two men convince them to go out with them on a boat to watch sharks from a diving cage. Lisa is hesitant but Kate persuades her by saying it would show Lisa’s boyfriend that she isn’t boring and make him regret breaking up with her.
The sisters arrive to the location where they were told to meet the locals. At first sight, the boat is lackluster and Lisa is immediately wary of it and its owner, Captain Taylor. Upon meeting the girls, the captain questions if the pair are experienced divers. Kate has been certified though Lisa knows nothing and has never been, so they lie and say they are both experienced. The dive cage is old and rusted, held up by thin chains and an old crane, adding further to Lisa’s apprehensions.
As they journey out to the cage diving spot, Captain Taylor baits the sharks. Lisa is skeptical about the entire dive, but Captain Taylor dismisses her concerns. The locals dive first in the cage. Unbeknownst to everyone on the boat, the cable supporting the cage starts to fray. The men return to the surface unharmed and Captain Taylor prepares to send Lisa and Kate down. While Lisa and Kate are in the cage, it breaks off of the boat, sending them to the sea floor – 47 meters down (154 ft.). This also takes them out of communication range. Kate swims up to forty meters to resume communications with Captain Taylor, who tells her Javier (Chris J. Johnson) will be coming down with a spare winch to pull them up. He also advises them to stay in the cage as the sharks are all around. Both women are quickly running out of air, but they wait in the cage until they see a torch in the distance. Thinking that Javier cannot see them, Lisa swims out to get him, as Kate’s air is lower due to the previous swim. Lisa swims towards the light, low to the sea floor to avoid shark attacks from the bottom. A shark tries to attack Lisa but she swims into a small grotto to avoid it. Continuing to go towards the light, she comes to a crevasse with no bottom in sight.
Swimming over the cliff towards the light she finds the torch but no Javier. While looking around, she gets confused about her distance from the cage and Kate. Just as Javier appears and attempts to usher her back towards safety he is attacked and killed by a shark. Lisa takes Javier’s spear gun and the spare winch, and swims back towards the cage. With the newly acquired torch as well as the spear gun Lisa swims up and notifies Taylor to pull them up. Disaster strikes as they are being winched to safety—the replacement winch line snaps and the cage descends back to the sea floor. This time, the cage lands on Lisa’s leg, pinning her down. Lisa and Kate panic as they realise they are now both low on options as well as air.