Lisa (Mandy Moore) and Kate (Claire Holt) are sisters vacationing in Mexico. Lisa has just broken up with her boyfriend but is keeping this a secret from Kate. One night, Kate wakes up and finds Lisa crying. Lisa tells Kate that her boyfriend left her because he got bored. Lisa tells Kate that he left because she's not fun enough and the reason she's on the trip is to regain that sense of fun that she used to have. Kate drags Lisa to a party where they meet two men, Louis and Benjamin.
After the party, Louis and Benjamin walk Lisa and Kate back to their room. Kate kisses Louis, but when Benjamin leans in to kiss Lisa, she backs away, telling him that she's not ready. As Benjamin leaves, Lisa tells him to wait as she runs up to kiss him, trying to embrace her fun side. Before they leave, Louis and Benjamin tell the two sisters about shark diving, which they do with their friend, Taylor (Matthew Modine). Kate urges Lisa to join her in trying it out. Lisa is hesitant at first but eventually gives in at the behest of Kate.
The next morning, the two sisters meet up with Louis and Benjamin who take the two to meet Taylor. Taylor is the captain of an old boat, which houses a rusty shark cage. Taylor asks the two sisters if they've ever been scuba diving before, which both claim they have. Louis and Benjamin are the first two to get into the shark cage. They get in and are lowered into the water. Lisa and Kate go into another room and get fitted into scuba gear.
Before they go into the cage, Taylor informs them of the equipment, which includes a meter that tells the user how much oxygen they have left. After Louis and Benjamin take their turn in the cage, Kate and Lisa get into it. They are lowered into the water. They don't see any sharks, so Taylor has his first mate, Javier, throw some fish blood into the water to attract the sharks.
It eventually does. However, the line holding the cage breaks, sending Lisa and Kate straight down the ocean, inside the cage. They eventually hit what appears to be the ocean floor, 47 meters down from the top of the ocean. Lisa is freaking out, and Kate has to calm her down. They are too far down for them to hear Taylor on their radio. So, Kate ends up having to leave the safety of the cage to swim to where she can get in touch with Taylor on the radio. She eventually reaches Taylor who tells Kate that he is going to send Javier down with a cable to bring the cage back to the surface. But, he warns Kate to stay in the cage because of the sharks.
Kate goes back to the cage and tells Lisa what Taylor told her. After some time has passed, they see a flashlight in the distance, which they deduce is Javier. Since Kate is running low on oxygen, Lisa decides to go and bring Javier to the cage. Lisa swims out to meet Javier but is suddenly ambushed by a shark. She is able to hide inside a small cavern of rocks, but the shark still tries to get her. The shark eventually leaves, and Lisa continues swimming towards the flashlight. She reaches a drop-off point, where the ocean continues heading down. But, she continues heading out to the flashlight, even though she cannot see anything below her. She eventually reaches the flashlight but does not see Javier. Javier appears out of nowhere, but suddenly he is attacked by a shark and killed.
Lisa swims away, but finds part of Javier's body and finds that he is carrying the cable that was going to be attached to the cage. Lisa takes the cable back and attaches it to the top of the cage. She swims up and tells Taylor that she has attached the cable to the cage and that Javier was killed. He tells her to get back to the cage.
Lisa swims back to the cage and Taylor begins raising the cage to the surface. They get about 20 meters from the surface when the cord snaps, sending the cage back to the bottom of the floor. However, when the cage lands, it pins Lisa's leg, trapping her. Noticing that she's running low on air in her tank, Kate swims back up to let Taylor know. He tells her that he is going to send another oxygen tank down to them. He also warns Kate that there is a danger of them possibly hallucinating if they are without oxygen for too long when they change oxygen tanks.
Kate sees the tank coming down and goes out to get it. A bag is attached to the tank with some flares. Kate gets the tank and is coming back to the cage when she spots a shark. She hides by some rocks and waits for it to leave. It swims by her, but due to how dark it is in the water, Kate and Lisa lose sight of it. Assuming it's gone, Kate decides to make a break for the cage. As soon as she does, the shark appears, killing her. She drops the tank, which lands near the cage.
Lisa grieves for her sister. A little while later, her oxygen monitor starts beeping, signaling she's running out of air. She sees the tank near the cage, but since her leg is pinned down, she cannot reach it. She sees a spear gun, which was also attached to the tank. She reaches out for it, but the trigger rubs against a part of the cage and accidentally shoots Lisa in the hand, drawing blood. Lisa screams in pain and begins looking at her wound.
She uses the spear gun to reach the oxygen tank. She eventually is able to get it and changes oxygen tanks, giving her more air. She begins hearing a voice on the radio and discovers it's Kate, she's still alive. Lisa decides to use her scuba gear to lift the cage, getting her free. She swims out of the cage and finds Kate, with a very bloody wound on her leg. They talk to Taylor and tell him that they are going to try to swim to the top. He tells them to go for it, but not to go too fast or else they will die, due to oxygen bubbles going straight to their brain. They use the flares to ward off the sharks, which works. As they get halfway, they have to stop to decompress for five minutes. As the flare burns out, Kate grabs another one but drops it. Lisa grabs the last flare, and as she lights it, four or five sharks are circling them. They continue to fend the sharks off with the flare. When they get to the top, they swim to the boat. Taylor, Louis, and Benjamin begin to help them back into the boat. Suddenly, Lisa is grabbed by a shark. She starts picking at its eye with her fingers, which works and it lets her go. Lisa and Kate are helped back into the boat.
Lisa begins looking at the wound on her hand and notices that the blood from it is blowing off into the wind, instead of flowing down her arm. She begins to hear Taylor's voice, and it is revealed that Lisa has been hallucinating everything after Kate's death. She is still pinned against the cage. Still hallucinating that she's talking to Kate on Taylor's boat, she says that they are finally free as she begins laughing hysterically. On the radio, Taylor tells her that she is hallucinating, but that the coast guard is there and they are coming down to help her. Lisa sees the coast guard coming down to help free her. As they help her out of the cage, she begins to come back to her senses. Tears roll down her eyes as she and the coast guard make their way back to the surface safely.
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