Near the Cayman Trough, the submarine USS Montana is noticing weird readings on their radar. Something strange appears to be coming closer, and suddenly the power goes off. The submarine starts shaking and the hydraulics are failing, so the crew uses auxiliary power to get the system back.
Unfortunately it’s too late and the submarine hits some rocks, which causes tons of damage and even triggers a fire. The crew loses control of the Montana and it keeps scraping against rock, making the damage worse. Water begins flooding the vessel, which is falling deeper and deeper into the ocean.
A buoy is launched right before the submarine finally crashes on the seabed and the water drowns the entire crew. Moments later, Doctor Lindsey and a SEAL team arrive at a ship supervising an underwater drilling platform, which they want to use it as their base of operations to investigate what happened to the submarine. In fact Lindsey designed the platform.
Currently many of the workers are handling some submersibles so the boss calls them in to inform them of the situation. The platform owners have agreed to help the government because they’re close to the sinking spot, so the workers’ new task is to search the submarine for any survivors. At first the team is skeptical but they quickly agree when they hear of a big bonus pay.
Foreman Virgil still protests because he thinks the team isn’t qualified, but the boss assures him the SEAL team will come along for extra protection. While the team gets ready to start the search, a storm arrives in the area. Lindsey calls Virgil to complain about the situation, but he ignores her.
It turns out they used to be married and don’t get along. Claiming she knows the systems better than anyone, Lindsey decides she’ll join the mission and pilots the submersible bringing down the Navy SEALs into the underwater platform. Once they get inside, a moment is taken to depressurize and Lindsey tells the men to be careful because one in every twenty people can’t handle it without going crazy.
Coffey is dismissive, but once they get inside, he hides a shaking hand. Then Lindsey begins to inspect every corner of the platform, pointing out anything that needs care. Virgil follows her around, wondering if she came for him only to end up arguing some more.
The frustration makes him throw his wedding ring in the toilet, however he immediately regrets it and puts it back on. Afterward both teams gather to discuss the plan while looking over the maps. The oil workers like to joke around and Coffey snaps at them, reminding them he’s in charge.
Another SEAL named Monk shows them the latest underwater breathing technology. He grabs Hibby’s pet rat and drops him into a small tub of fluid. After a few seconds of anxiety, the rat can breathe normally.
Eventually the divers finally descend to start searching the submarine. After making sure the area is safe, they launch the ROV or remotely operated vehicle which is controlled by Hippy. They also scan for radiation and don’t find anything, meaning the submarine’s warhead is intact.
The divers enter the submarine and start searching for survivors, finding nothing but broken things at first. The hatch at the end of the corridor is stuck so they work together to open it, and a body floats out. When the oil workers freak out, Coffey has to remind them this was expected and urges them to keep moving.
The group keeps searching the submarine, finding more and more bodies along the way. Coffey decides to split the team in two to cover more ground and they agree to head back in fourteen minutes. Jammer sees crabs coming out of a body and has a panic attack, so Virgil comforts him and tells him to stay put while he searches the last room.
A few minutes after Virgil leaves into the other room, Jammer’s lights start to fail and he can’t contact Virgil on the radio. He pulls at Virgil’s rope to get his attention, only for the rope to break. Suddenly a glow appears around him and he sees a strange creature floating in front of him.
A terrified Jammer tries to escape, accidentally damaging his diving gear in the process. Virgil finally hears trouble and swims back to find Jammer convulsing. The others also come and notice that the diving gear is giving him too much oxygen, so they cut it off and Jammer passes out.
The group then works together to move the body out. In a submersible, Lindsey also loses the lights and sees the same strange creature swim by very quickly. After it’s gone, the lights return to normal and Lindsey is called to pick up the team.
Minutes later the team is back on the platform and the SEAL medic confirms Hammer is in a coma, so they don’t know when he’ll wake up. Since there’s nothing on the cameras, Virgil thinks that whatever Hammer saw was a hallucination caused by the oxygen issue, but Lindsey swears she saw something too. Coffey reports to his boss, who tells the SEAL team to recover the warhead in secret.
Meanwhile the oil workers watch the news and learn the Russians are being blamed for the sunk submarine. At that moment the oil workers discover that Coffey and his men are leaving in one of the submersibles without permission, ignoring Virgil when he yells at them about the potential dangers. On the surface, the storm gets worse and it’s discovered there’s a hurricane coming.
The oil workers in the ship want to evacuate, but they can’t do it while the SEALs are out there exploring. In the submarine, Coffey and his men are carefully going through the different steps to free the warhead. As soon as they’re done they bring it into the underwater platform, and Lisa goes out in a submersible to secure the rig because the storm is causing the whole structure to shake too much.
It’s taking her a few tries, and in the ship the workers notices the rig is swinging out of position. Underwater, the rig begins sliding on the seabed, pushing Lisa’s submersible out of the way. Virgil tries calling the people above but the worker in the crane can’t get any messages.
Suddenly the rig gets stuck and the crane swings again, breaking through the glass before it breaks and collapses. As the crane sinks into the ocean with the worker in it, Virgil’s team is warned about it so they can run to safety. Part of the crane hits Lisa’s submersible, but she manages to move away before it can cause damage.
The crane lands next to the underwater platform and for a second everyone is relieved, but then it continues to slide and falls over a trench. Since it’s still connected, the platform gets dragged as well. The electrical connections begin to spark inside and start a fire while everyone tries to hide, only to learn the drill room is flooding.
Soon more sparks trigger even more fires and a hatch door blows up, hitting and hurting Monk. Lindsey puts on a mask to go down and fight the flames with an extinguisher. Hippy runs to the drill room with his rat protected in a bag, closing the door to seal the room.
He tries to stop the moving submersible to no avail so he leaves the room, dropping his rat in the process. The little guy is about to get crushed, but Hippy jumps in and grabs him, moving out of the way right before the submersible crashes against the exit. Then it falls back into the pool, crushing a man underwater.
At that moment the platform stops on the edge of the trench in a very flimsy position. A huge amount of water bursts in and a few workers try to run away, however the doors automatically close and cause the workers to drown inside. Virgil rushes to another door that is also closing but it’s too late and more of his friends die in the flood.
The water comes for him next, so he runs to the following door and puts his fingers in before it closes. This room is about to get fully flooded as well, but luckily two workers arrive and push the door open. Then Virgil and his friends run into the following room, closing the door behind them before the water can follow them.
Afterward the few survivors gather in the main area. The SEALs take care of Monk's wounds while the oil workers try to use the radio, but it isn’t working. Lindsey checks the generator and explains there isn’t enough energy for the heaters, so in a few hours the platform will become a fridge.
They have around twelve hours of oxygen left and since the storm is probably going to last longer than that, the chances of rescue are low. Remembering there are some oxygen tanks in the wrecked module, Lindsey suits up and heads out to find them with Hippy sending the ROV behind her. Eventually she finds the tanks near the edge of the trench.
As she works on getting them out of the module, the connection starts failing and Hippy loses contact with the ROV. Lindsey loses the lights again and suddenly the strange creature appears near her. She watches in as the little creature floats away, only to bring out a big glowing thing.
This time Lindsey gets to touch it before it swims away. She grabs the ROV’s camera and takes pictures as the little creature follows the big one out. When she returns to the platform, everyone takes a look at the picture, which is a pink blur.
Lindsey tells them the creature is definitely not human but it’s intelligent, possible even alien. Most of the workers get excited but Virgil scolds her, saying she’s creating hysteria over something that may not be real. In the meantime, the SEALs are working on the warhead, which is seen by Hippy when he uses ROV to spy on them.
He immediately informs Virgil and Lindsey, who furiously runs to the SEAL’s room and hits the door with an extinguisher until they open it. She begins yelling at the soldiers for bringing this in without informing the oil company and a man grabs her to shut her up, however Virgil calls his team and everyone rushes in to help her. After the oil workers are gone, Coffey reveals he has a gun and says they can’t trust anyone, showing signs of growing delirium.
Afterward Lindsey discusses with Hippy the possibility of sending the ROV out to find proof of the creature, hoping to prove it’s not the Russians attacking. This is overheard by Coffey on the security cameras and gives him an idea. Then everyone decides to rest for the night, unaware something is getting inside through the pool.
The mysterious creature begins sneaking around, opening doors as it checks on each human. At that moment the oil workers wake up and see the creature, which is serpent-like and made of water. It comes as close as possible and begins copying Lindsey’s facial expressions to then do Virgil’s face.
When Lindsey touches it, her finger goes through and picks a drop, which she tastes to confirm is seawater. Next the creature rushes to check on the warhead and this time the SEALs see it too. Coffey closes a door and cuts the creature in two, with the end half falling into the poodle while the main bodies goes back into the pool to leave.
Afterward the group discusses what they saw and Lindsey theorizes the creature can control water and made the serpent, which Hippy thinks was the alien’s version of a ROV. While Coffey listens to all this, he uses a knife under the table to deal with his anxiety. Suddenly he runs out of the room and his friend follows him, only to find him putting together his weapon.
Coffey grabs him as he says they must protect the world from an alien attack and gives him the gun. Moments later Hippy discovers a soldier attaching the warhead to one of the ROVs to send it after the creature. When he tries to step back, an armed Coffey blocks him and drags him back to the main room.
Coffey and his partner keep the oil workers at bay by pointing their guns at them, then Coffey brings Monk out of the infirmary while Hippy tells the others about the warhead in the ROV. Lindsey tries to make Coffey see reason but he responds by pushing her against the wall and covering her mouth with tape. Next the SEALs lock the oil workers up in the infirmary while Monk protests against Coffey’s absurd plan, but he’s ignored.
After Coffey heads off, Jammer wakes up and brings the SEAL guard down before freeing his friends. However when they try to go after Coffey, they discover he’s jammed the door leading to the pool. Virgil and Catfish decide to swim out to look for an alternate entrance, ignoring Lindsey’s warning about the freezing temperature.
The duo quickly finds another hatch and gets in, but the following door is stuck and they can’t open it. Catfish is out of air and needs some time to recover, so Virgil continues to swim alone. This time Virgil manages to get back inside by entering through the pool, noticing Coffey very slowly handling the ROV’s chain.
He grabs a widget and tries to make a surprise attack from behind, however Coffey hears him and takes out his gun. When he tries shooting, nothing happens because Monk removed all the bullets. Then Virgil hits him and Coffey retaliates with a kick, triggering a hand-to-hand fight.
Coffey has a knife and attempts to stab a few times before Virgil hits him again, causing them to fall in the water. The men fight for the knife and Virgil bites Coffey to escape his hold, so Coffey pounds him on the wall a few times before he starts choking him with some cables. At that moment Catfish appears and punches Coffey, who decides to jump inside a submersible with the ROV and the warhead attached.
As it goes down the pool, Virgil and Catfish jump on top of it but fail to open the hatch, so they get off to avoid drowning. Next Catfish opens the door from the inside to let the others in and uses Monk’s gun to shoot into the water to no avail. Virgil suits up and dives into the pool, managing to grab onto Coffey’s submersible at the last second.
He takes some rope from the tool box and ties it to the ROV right as Coffey releases it. Swimming back to the platform, Virgil ties the other end of the rope to it, so Coffey tries coming after him with the mechanical arm. Instead he crashes the submersible against the platform while Virgil swims to hide.
A chase ensues until Lindsey appears in another submersible and crashes against Coffey’s, making it catch on fire. While he’s busy using the extinguisher to put the flames out, Virgil gets inside Lindsey’s submersible. The ROV is still pulling and the rope comes loose from the platform, so the couple tries to grab it.
At that moment Coffey hits them with his submersible and they lose the rope, so the ROV floats away into the trench. Next Coffey begins crashing against Lindsey’s submersible over and over with his own, so Lindsey hits some rocks on purpose to make them fall on top of Coffey’s vehicle. This allows her to approach it from the side and drag it through the seabed until she finally pushes it over the edge of the trench.
Coffey gives them a final crazy look before his submersible falls, dying when the glass breaks under the deep pressure. The fight has caused severe damage to Lindsey’s submersible too and now water is getting in. Soon a short-circuit happens and leaves the submersible with no power or radio.
The couple starts arguing over the diving suit, thinking the other should get to swim out. Lindsey points out that Virgil is a better swimmer, so she should carry her out. She’ll pass out under hypothermia but he’ll be fast enough to revive her in minutes.
Seeing no other choice, Virgil agrees with a kiss before putting on his helmet. Lindsey panics before going underwater, immediately passing out. Virgil cries out before he starts swimming, using the suit’s radio to tell the others to emergency kit ready.
As soon as he makes it through the pool, the entire team surrounds Lindsey’s body and starts working on waking her up. Defibrillation doesn’t do anything and Virgil desperately starts using CPR. After several minutes the others give up, however Virgil keeps going and even slaps her as he yells at her.
This finally wakes her up and everyone celebrates. Now they need to do something about the warhead. The team puts Virgil inside a special suit and gives him special contacts to protect his eyes since he’ll be using Monk’s breathable liquid to go out.
When they close the helmet and the liquid starts filling it, Virgil’s body reacts very violently, but soon it gets used to it and he can breathe. After testing the suit’s keyboard for messages, Virgil dives into the pool. With a ROV in hand, Virgil jumps into the trench and sees all the things the fell there earlier.
As he falls more deeply, his hands start shaking and he informs the others, so Lindsey keeps on talking to him on the radio to keep him sane. As Virgil’s messages become gibberish, the ROV’s lights blow up. The shockwave pushes Virgil down faster and he uses a flare to try to stay orientated.
Lindsey continues to talk to him and for a moment she thinks they lost him, but eventually a message arrives saying he’s made it. Virgil is now surrounded by a mysterious light that allows him to find the warhead, and Monk starts instructing him on how to disarm it. Very carefully, Virgil cuts the wires, and thankfully nothing explodes.
The team celebrates until they receive bad news: Virgil only has five minutes of oxygen left, and the trip back would take him thirty. Lindsey freaks out and tells him to try anyway, but Virgil admits he knew this would happen and says he loves her. Minutes pass and Virgil waits for the end, but at that moment the light comes closer and the creature appears in front of him.
It reaches out with its small hand and once Virgil grabs it, it takes him into its underwater city, which glows with neon colors. Virgil is about to pass out when the creature controls the water to empty the room so he can breathe. More creatures appear behind him and play his message to Lindsey on the water as a way to explain why they saved him.
Suddenly the storm completely disappears from the surface and the ship can finally contact the team underwater, promising rescuers will go for them soon. At that moment a message from Virgil appears on the screen saying he has new friends. The platform starts shaking and a glow appears outside as the alien city appears under them.
On the ship, the crew sees strange readings all around them. Everyone runs outside and watches in awe how the alien city emerges from the water, rising the entire rig with it. The team finally comes out and they realize the creatures did something special to them because they didn’t die over the lack of decompression.
At that moment Virgil also appears and runs to reunite with Lindsey, calling her his wife before kissing her.
