Four teen girls diving in a ruined underwater city quickly learn they’ve entered the territory of the deadliest shark species in the claustrophobic labyrinth of submerged caves.
VERY SHORT VERSION
Lisa (Mandy Moore) finds Kate (Claire Holt), who has been badly injured and is nearly out of oxygen, and manages to drag her to the surface where, despite several shark bites, they are pulled into the boat. It is then revealed that Lisa was hallucinating and is still trapped in the shark cage 47 meters down. Kate is presumed dead. The Coast Guard finds Lisa, frees her, and guides her up to the surface as the credits roll.
Introverted outcast Mia joins her stepsister Sasha and her friends Alexa and Nicole on a spelunking trip to an old Mayan cave. There, they are stalked by a shark in the catacombs. With their oxygen running out, they struggle to find a way out of there safely.
Mia’s father Grant is nearby with two assistants on an expedition. The assistants are eaten by the shark, but Grant tries to help the girls get out using a harness that leads out of the cave. Alexa starts to climb, but Nicole tries rushing up in a panic, causing the harness to come loose and leads her to fall and become shark food. Grant gets killed just moments after they resolve to swim out of a nearby gap.
The remaining girls then try swimming to the surface, but Alexa drowns when her oxygen runs out. Mia and Sasha make it out and see a shark-sighting boat that can help them. Unfortunately, the crew members are dumping chum into the water, luring more sharks there. The girls get the attention of the people onboard and are nearly killed, but thanks to Mia’s quick-thinking, she saves herself and her sister from the sharks.
Lisa (Mandy Moore) and Kate (Claire Holt) are sisters vacationing in Mexico. They meet two men who invite them out to see great white sharks from a cage. Lisa doesn’t want to do it (and does not have SCUBA certification), but as a skilled SCUBA diver, Kate promises to take care of Lisa.
They meet the guys on the boat of the captain, Taylor (Matthew Modine), and go out on the water. The guys take a turn without a problem, but when the girls get in the rope breaks and they plummet 47 meters down to the bottom of the ocean.
As they are out of communication range, Kate has to swim a couple of meters above the cage in order to talk to Taylor through her suit’s comm unit. Taylor promises to send his employee, Javier, down with a spare cable to pull the cage back up, but when Kate and Lisa finally see his flashlight it does not move in their direction. They assume he can’t find them.
Since Kate is very low on air, Lisa swims out to find Javier. She locates the flashlight but Javier is nowhere in sight. Lisa loses her way and begins to panic when Javier yells for her to hide–just in time for a shark to kill him. Lisa manages to hide until all seems calm. She eventually crawls out, locates the cable to pull them up and a spear gun, and Kate guides her back to the cage by making noise when Lisa shines the flashlight in her direction.
Kate’s air is now perilously low and Lisa is getting down there as well. Lisa swims up until she is in range of Taylor and informs him that Javier is dead but they have the cable. Taylor pulls the cage back up but as they are getting close to the surface the cable breaks and they plummet back to the bottom.
Lisa’s leg is now trapped under the cage. Kate swims up to tell Taylor they are still alive and he promises to drop down extra air tanks while they wait for the Mexican Coast Guard, which should arrive in about an hour. Significantly, Taylor warns Kate that switching to a second tank increases the risk of nitrogen narcosis, which causes hallucinations.
When the tanks are dropped, Kate swims out to retrieve them and finds Taylor has sent flares as well. Kate switches her tank and narrowly makes it before running out of oxygen. Just as she arrives back at the cage, a shark swoops in, grabs her, and they both disappear.
Lisa’s replacement tank has fallen just out of reach. She cuts her hand on the spear gun trying to use it to pull the tank toward her on her first attempt. She keeps trying and manages to pull the tank to her just as she runs out of air. With no oxygen, she struggles to get it attached and finally does, breathing in relief.
Shortly after, Kate begins talking to her through the comm system and tells Lisa that she is alive but badly injured and her blood is attracting more sharks. Lisa realizes she can use her BCD to lift the cage and free her leg. She finds Kate and begins to help her swim up toward the surface. Kate reminds Lisa that they cannot swim quickly or they will get the bends and die, and Kate helps Lisa pace them as they swim up–using the flares to scare off sharks as they approach. They get into range of Taylor and he reminds them to stop at the twenty-meter mark so they can adjust to the pressure to avoid the bends. While they wait, Kate accidentally drops a flare and when she manages to ignite another the sharks have surrounded them, so they swim up to the surface immediately and make a break for the boat.
They catch a life preserver Taylor throws out, but a shark grabs Lisa and drags her under. She fights it off by gouging its eye with her fingers, and the sisters are hauled into the boat. As Taylor and the guys administer CPR, Lisa looks at the wound on her hand from the spear gun. It is still bleeding, but the blood is rising out of the wound as if she were underwater. It is then revealed that Lisa is still trapped in the cage, having hallucinated the whole escape. She is still hallucinating when the Coast Guard team finds the cage and pulls her toward the surface. As they approach the boat, Lisa seems to realize Kate is not with them and begins to cry.
The film starts at a girls school in Yucatan, Mexico. Mia (Sophie Nelisse) is pushed into the pool by mean girl Catherine (Brec Bassinger). Mia’s stepsister Sasha (Corinne Foxx) doesn’t help her and even denies being her sister. They are picked up by Sasha’s mom Jennifer (Nia Long), who married Mia’s dad Grant (John Corbett). Sasha tells her parents none of the other girls like Mia, while the latter reiterates the former’s claim that they aren’t sisters.
Over a meal, Grant shows Mia a shark tooth he found on one of his cave-diving expeditions, and he gives it to her, as she shares his interest in discoveries like this. Mia and Sasha are told that they are set up to go on a boat to see the sharks, which doesn’t make Sasha happy since she was planning on hanging out with friends.
Grant and his two assistants Ben (Davi Santos) and Carl (Khylin Rhambo) drop the girls off as they set out on another cave-diving venture nearby. The girls see that Catherine is there with her friends. Sasha’s friends Alexa (Brianne Tju) and Nicole (Sistine Stallone) drive up to get Sasha out of there. She reluctantly convinces Mia to come along since she knows she would rather be anywhere than stuck with Catherine. The girls give Catherine the finger and head off.
The girls arrive near a cliffside by a cave and jump in the water. Mia actually starts to have fun with Sasha and her friends. They then grab scuba gear and decide to go exploring nearby since it is in the ruins of an ancient Mayan temple. They swim through the cave and find many old carvings, as well as many skeletons. Nicole sees a blind Mexican Tetra swimming nearby. Alexa notes that it has been there a while and doesn’t need eyes since it has adapted. The Tetra swims toward Nicole and frightens her when it screeches, causing her to bump into columns that causes a commotion, splitting the girls up. Mia and Alexa find each other and then come across Ben, which means Carl and Grant are not far. A shark then pops out and kills Ben, causing the girls to flee. They swim into Sasha and Nicole again, and they tell them about the shark.
The shark attacks the girls, but since it is blind, it uses its hearing senses to find them. It crashes into the columns and leaves the girls trapped for a moment. Once they get free, Mia and Sasha suggest they find Grant and Carl using a guideline that will lead them to where they were working. The girls swim through catacombs and see that the line ends there. Regardless, they continue to swim forward while also minding the fact that the oxygen in their tanks is running low. The shark then pops out again, forcing them to hide behind a cage for protection. They escape by making it through a passageway too narrow for the shark to fit through. They make it into an air pocket to get oxygen back while trying to figure out their next move. Mia swims down and sees a gap that she can fit through and she goes for it.
Mia swims nearby where Carl is working and listening to music. Unfortunately, he doesn’t see the shark and promptly becomes its next meal. An emergency alarm then goes off as he is being shredded. Mia hears more music and finds herself alone and scared as she comes across Carl’s remains. She eventually finds Grant, and the two go together to find the other girls. They are attacked by the shark again, and they fend it off using the sounds of a nearby bell, plus a blowtorch. The five reach the surface in the cave and use the alarm to scare the shark off temporarily.
Grant points out a harness at the top for them to climb out of. Alexa starts to ascend first, but Nicole panics and jumps up to try and climb ahead of her. The harness starts to come loose with their combined weight, and although Nicole reaches the top, she slips and falls back down into the water before being quickly devoured.
Grant and the girls then figure they need to swim to a nearby gap to get out of there, but the shark gets Grant moments later, leaving Mia and Sasha devastated. They swim together with Alexa to try and get out of there. They come across a current that pulls Sasha away from Mia and Alexa. The shark finds them again and attacks, but Alexa evades it. However, she is out of oxygen, and she soon drowns.
Mia finds Sasha and they end up swimming to the surface. They find a boat and try to signal it. It turns out to be the boat that they would have gotten on to see the sharks. Someone on the boat is listening to music and can’t hear the girls calling, and he dumps chum into the water for the sharks. This lures a few more sharks in that direction. Mia and Sasha bang on the glass where the guests can see them, and they run to alert the crew. Mia makes it up on the boat, but Sasha gets pulled in by a shark. Mia grabs a flare gun and jumps in to save Sasha by firing the flare at the shark’s face. Just as they get back, another shark grabs Mia and pulls her in. She frees herself by taking the shark tooth and slicing across the shark’s side to injure it, freeing herself. The people on the boat pull Mia in before another shark can get her.
The girls receive medical attention, but Mia and Sasha are just glad that they still have each other.