Lisa (Mandy Moore) and Kate (Claire Holt) are sisters on vacation in Mexico. Lisa’s boyfriend broke up with her because “she made the relationship boring”. Kate tries to cheer Lisa up and takes her out drinking and dancing at 4:00 am. The sisters meet a pair of local men at a bar and they convince them to go out with them on their boat to watch sharks from a diving cage. Lisa hesitates but Kate persuades her saying it would show Lisa’s boyfriend that she isn’t boring and make him regret breaking up with her.
The sisters arrive at the boat dock. 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 asks if the sisters are experienced divers. Kate is a certified diver but Lisa is new to diving, so they lie and say they are both experienced. The dive cage is old and rusty and hangs from a thin chains from an old crane, adding further to Lisa’s apprehensions.
As they reach the 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 and Captain Taylor prepares to send Lisa and Kate down. When Lisa and Kate are in the cage, the cable breaks and the cage sinks to the bottom, some 47 meters below the surface and out of communication range with the boat. Kate swims up seven 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 circling. 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, close to the bottom to avoid shark attacks from the below. 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 not 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 also snaps and the cage sinks back to the bottom. The cage lands on Lisa’s leg, pinning her down. Lisa and Kate panic as they realize they are now both out of options and air.