A crew of aquatic researchers work to get to safety after an earthquake devastates their subterranean laboratory. But the crew has more than the ocean seabed to fear.
- The film begins with images of the Mariana Trench and describing Tian Industries' intention to drill seven miles to the bottom for resources. Norah Price (Kristen Stewart), a mechanical engineer at the Keppler 822 Station, is brushing her teeth when the ceiling starts to trickle water. A large earthquake hits, and the Keppler starts to suffer catastrophic breach from the pressure. Price tries to wake everyone up but only she and another worker named Rodrigo (Mamoudou Athie) are able to escape the area and close it off, preventing further damage to the Keppler. Trying to find a way off the Keppler via escape pods, they encounter Paul (T.J. Miller) under the rubble and crawl through an access point to reach the escape pod bay. All of the escape pods have already been deployed, with Captain Lucien (Vincent Cassel) being the only person in the area when the three arrive.
The surviving crew reaches a control base and find biologist Emily Haversham (Jessica Henwick) and engineer Liam Smith (John Gallagher Jr.). Trying to reach out via radio, they are unsuccessful in making contact with the surface. Lucien then recommends that they put on pressurized suits and walk one mile across the ocean floor to the Roebuck Station 641. They start to emerge from the station into a cargo elevator, but Rodrigo's helmet is faulty and cracks from the pressure, killing him instantly. The surviving crew see a distress beacon from one of the escape pods below, and Smith and Paul go outside to investigate. As Smith and Paul arrive at the location, the pod has been ripped open and a body lies in the rubble. Paul inspects the body, only for a hatchling creature to emerge from it and attack him. Smith is able to shoot and kill it with a bolt gun, and they bring it inside. Studying the creature's body, Haversham realizes it is a part of a previously undiscovered species. The five begin to set out towards the Roebuck Station, but as they are walking, the Keppler explodes, causing debris to fly towards them. Smith is hit by the debris, but Price and Lucien manage to save him and bring him inside. They are able to make it to the meet-point station by taking an access tunnel, allowing them to charge and clean their suits, but find that Smith's oxygen source is badly damaged from the debris. As they make their way through the access tunnel, Paul is attacked by a creature and is dragged underwater before being ripped out of his suit and killed.
Before leaving the access tunnel, the team discover that Smith's damaged oxygen pod will cause him to have to breathe in toxic fumes from the explosion. Unwilling to leave another crew member behind, Price, Lucien and Haversham agree to help him walk, dragging him if they have to. The remaining four crew members press on and start their walk across the ocean floor, but another creature appears, dragging Smith into a cave. Lucien manages to pull Smith out, but then tries to go for Smith's bolt gun, allowing the creature an opportunity to drag Lucien up through the water and away from the other three. Price follows the creature and Lucien and is able to free him, but Lucien sacrifices himself so that Price may escape the increasing change in pressure. Price ends up at the abandoned Sheppard Station and finds that Lucien worked there beforehand, causing suspicion as to what Lucien really knew about these creatures and their involvement with the drilling stations. Price tries to make contact with Smith and Haversham, but to no avail. She then gets a new pressurized suit and leaves the Sheppard, continuing toward the Roebuck. Walking along the ocean floor, Price reunites with Haversham, dragging Smith towards the Roebuck Station, and proceeds to help. As they enter the station, they notice that there is a nest of creatures hanging from the ceiling and try to sneak by, but Haversham's suit, low on oxygen, begins to make noise, causing one of them to wake up and attack Price. Price is partially swallowed by the creature, but is able to kill it and break free, allowing them to continue their way into the Roebuck.
The three are able to reach the escape pod bay, but Price discovers that only two work, with a third being damaged and unusable. Price and Haversham manage to get an ailing but still alive Smith into one of the pods, and Price eventually convinces Haversham to take the last one. As this is happening, a gigantic version of the creature that attacked them emerges from around the Roebuck, surrounded by even more creatures. The creatures start to follow the two escape pods, while the gigantic one begins to attack the Roebuck. Price, knowing she is already going to die, raises the energy levels of the core engines so that they may explode, killing all the creatures in the vicinity and allowing the escape pods to reach the surface safely. The film ends with images of newspaper articles depicting Tian Industries and their attempts to cover up the incident, silencing Haversham and Smith. The company goes on to say that they intend to expand their drilling efforts.