In the Indian Ocean off the coast of Somalia, a ship belonging to an oil company is full of
workers that do maintenance on underwater oil and gas pipelines by diving into extreme depths. One
morning, an alarm begins blaring through the ship, so Engel grabs the little necklace he
keeps as a memory of his dead wife and goes to check out what’s going on. When he makes it to
the comms room, he finds boss Karsen arguing with their superiors on the phone and Mitch explains
that the ship is turning because they’ve found a low-pressure read on the pipeline and manifold.
Karsen wants to send a team underwater and ignores Engel’s protests that a storm is coming.
Since most diving teams have already gone home, they’ll have to gather whoever they can find,
including a very young employee. Engel protests again, pointing out that the boy hasn’t gone
below thirty meters, but Karsen sticks to his orders and puts Mitch in charge. Afterward,
Engel goes to tell young Jones the news, and the boy’s actually excited to finally be going
underwater. Mitch goes looking for Hurst and is disturbed to discover how much he’s been drinking.
Hurst points out that his shift is over, but he accepts to go when Mitch offers double pay. Then
the team gets ready, and Jones gives his pregnant wife a call to tell her he’ll be a day late,
but unfortunately Mitch doesn’t find anybody at home. Moments later, the team finally enters the
diving bell and begins descending into the ocean. Engel notices Jones has brought a jar of hair gel
and tells him to open it, which ends up spilling all over Jones’ clothes because of the pressure
change. The deeper they go, the more Hurst begins feeling the alcohol withdrawal and his hands shake
while merely putting on his seatbelt. When the team finally makes it to the bottom,
Jones stays on the bell while the other three go out to fix the pipe. The three men work hard
on getting the repairs done, but it will take a while. Jones tries to keep himself entertained
by watching a beautiful creature swim by and staring at a picture of his wife. Suddenly, the
bell starts shaking like crazy and Jones rushes to get in contact with the ship. Apparently the storm
has finally arrived, but before Jones can ask for more details, the ship’s comms go off. When Jones
tells the others what happened, Hurst refuses to finish the job and swims back to the bell,
leaving Engel and Mitch to finish the repairs. At that moment the bell starts shaking again,
and Hurst urges his friends to hurry back. Luckily they finish the repairs just in time and return
to the bell, where they finally make contact with the ship again. Karsen says they were hit,
but Mitch still asks him to bring the bell back to the surface. The bell slowly begins to ascend,
but the higher it goes, the more it shakes, and soon they lose contact with the ship again.
Suddenly the bell starts dropping at a high speed and the team has to brace for impact. As
soon as the bell hits the bottom of the ocean, the alarm goes off, and Jones screams because he got
hurt. Mitch and Hurst immediately hold him down while Engel puts his knife inside Jones’ mouth:
it turns out he had pressure caught in a tooth cavity under the filling, but the poke of the
knife quickly takes care of it. While Jones takes a moment to recover and the system stabilizes,
Mitch tries to contact the ship again to no avail. Engel decides to go outside to check the damage,
discovering that the damage is just some scratches and that the beacon is still working. Hurst checks
the inventory and finds blankets and first aid, but no survival suits. Mitch points out the backup
heating is working, so hopefully that will be enough. Afterward, Engel continues to swim to
check on the umbilical cables that connect the bell to the ship. He follows the trail until he
discovers a disturbing sight: the ship has been destroyed by the storm and he’s looking at the
sinking parts. To make matters worse, Engel finds all the bodies of the crew, including Karsen.
Engel returns to the bell and drops a piece of the ship to prove it’s been sunk. At first Mitch
refuses to believe it and asks Jones to try the comms again, but Engel mentions the bodies and
yells at him to make him see the truth: they are stuck here with limited oxygen, so they don’t
have much time left. After a moment of silence to let the news sink in, an argument begins,
which is made worse when Engel explains that the company is in lockdown and nobody will come.
Hurst and Engel grab each other, ready to add punches to the fight, but Mitch quickly separates
them and asks everyone to calm down in order to save oxygen. Then Mitch remembers they have the
manifold location of the pipe from when they came down to fix it, so he writes it down and gives it
to Jones to transmit it through the comms every fifteen minutes, hoping any passing ship will get
the message. Engel thinks they should detach the clump weight to make the bell float again so they
can be seen, but Mitch doesn’t let him, pointing out that pressure is equalized inside and out, so
if they rise it could kill them. At that moment, one of the oxygen tanks gets loose, and the bell
starts losing air, so the alarm goes off again. The team rushes to get some masks, especially for
Jones who isn’t used to this and is already feeling dizzy. Engel also goes down because
of his age and takes the other mask while Mitch stabilizes the bell and notices they’ve lost two
hours of breathing gas. Another argument ensues as Hurst wants to go looking for the ship wreckage
for a new tank and Engel insists on making the bell float, but Mitch insists that it’s too risky
and they should just wait. Jones continues to try the comms and suddenly they get an answer from a
Chinese fishing ship. Unfortunately because there aren’t Chinese citizens in the bell, they refuse
to offer help and hang up. Afterward, Hurst and Mitch suit up to go double-check on the bell
damage before they lose another oxygen tank. Engel watches them go and begins thinking about the day
he was out on a trip with his now-dead wife, and Jones guesses Engel has no hope of getting
rescued. Meanwhile Hurst and Mitch make sure the cylinders and the valves are well adjusted,
but they also discover that there’s a crane block on top of the bell. While they work on making the
valves extra secure, the crane block falls and crashes against Mitch, knocking him out. Hurst
panics and rushes to help him, only to discover Mitch’s umbilical is snagged. He tries to detach
him, but his shaking hands are having trouble, so Engel ends up jumping into the water with no suit
at all. Engel’s experience allows him to quickly detach Mitch from his umbilical and together with
Hurst they drag him back into the bell. As soon as they make inside, they remove Mitch’s helmet and
he immediately wakes up as he takes deep breaths. Then he proceeds to scold the heck out of Hurst,
saying his diving days are over because his hands are useless now that he’s an addict.
Hurst apologizes, but he also points out that the mechanism had been broken and that he only drinks
because of the isolation caused by eighteen years of being a diver. A few hours pass, and the bell
loses half the air. Engel insists they should do something, but Mitch is stubborn and still
orders them to wait. Moments later, Engel and Jones are sleeping while Mitch throws himself
into praying. He’s about to fall asleep too when he hears radio static, but when he runs to check,
he sadly discovers it’s a false alarm. Then he goes to check on Hurst, only to find him gone.
Mitch wakes the others up and uses the radio to call Hurst, who announces he’s going to fix
things. It turns out Hurst has put on a suit and is following the ship’s umbilical to try to find
extra oxygen tanks. Mitch orders him to come back, but Hurst ignores him and keeps talking about his
old diving experiences. However the further he goes, the worse he's affected by hypothermia and
soon he enters a delusional state, which makes him ramble about his family. Soon Hurst begins
complaining about how hot he feels and falls to the ground, dragging his body on the sand as he
struggles to breathe. In the bell, Engel points out they should cut the gas since Hurst is going
to die anyway. Engel thinks they should save oxygen and Hurst should die quickly instead of
painfully slowly. Mitch's disturbed by the idea since he doesn’t think they have the power to
choose who dies, and this triggers yet another argument. While Engel and Mitch fight, Jones
makes the choice and presses the button to cut the gas, and soon they stop hearing Hurst’s breathing,
which confirms he’s dead. What they don’t know is that he had been two steps away from finding the
ship’s oxygen tanks. To say goodbye to Hurst, Mitch offers a prayer and throws Hurst’s other
umbilical end into the water. Moments later, Jones decides that since he didn’t get to dive
on the job, he’ll go swimming now. He jumps into the water and is surprised to find his wife there,
looking beautiful with no clothes. The two of them reunite and start kissing, but when things start
to heat up, blood begins coming out of their mouths. Jones panics and tries to swim again,
but his wife keeps him in her hold firmly as blood continues to come out. Suddenly, Jones opens his
eyes and realizes it had all been a nightmare. When Mitch checks on him, Jones starts to accept
he isn’t going to make it and that his wife will have to have the kid alone. He wonders what
fatherhood is like, and Mitch tells him a story of how amazing it felt to hold his baby for the first
time. At that moment, the heater starts failing, which will add the potential of freezing to death.
Luckily Jones knows how to improvise and uses a torch on a tank they fill with water to make a
homemade steam boiler. Now they have a little over an hour left of air, so Mitch decides to write a
letter to his family, apologizing for not leaving the job when they asked him to. Then he saves it
in a sealed plastic bag so it can be found with his body later. Seeing this brings memories back
to Engel again, but the moment is interrupted when the comms suddenly come to life: it’s a navy
frigate, that came after the Chinese fishing ship passed them the bell’s message, and they confirm
that the company never contacted anyone. This new ship will start looking for them, but it may take
hours to find them because they don’t have the exact coordinates. Since it looks like they’ll
still die, Engel offers an idea: the drywall chambers on the pipelines carry spare cylinders,
so if they mix the gas with their remaining helium, they might buy enough time. Mitch
is hesitant because they’ve drifted for miles and there’s no way of knowing where it is, but
Engel points out they don’t have much of a choice anyway. Mitch approves of the plan and Engel suits
up to start searching. Once he finds a pipe and tells Jones the number over the comms, Jones looks
at the map and guides him to the closest possible weld chamber. By walking further, Engel uses the
suit’s gas to the max and runs out of heat. Mitch tries to tell him to come back, but Engel refuses
since they’re going to die anyway. Jones continues to give him directions through the comms and Engel
finds a lot of bodies, but he also finds the weld chamber at last. When he’s about to reach for it,
he discovers he’s at the end of the umbilical line and he can’t advance anymore. Seeing no
other choice, Engel decides to detach to reach the tanks, ignoring Mitch’s warnings and losing
the comms. There’s almost no air left in the bell, so Mitch decides to put on the other suit and go
after Engel. He finds the detached umbilical and fears the worst, but after walking a bit more,
he finds Engel unconscious on the ground with a tank next to him. Pushing his own umbilical to the
extreme, Mitch manages to grab both Engel and the tank and bring them back to the bell. Jones helps
them up and immediately takes Engel’s helmet off, but the alarm suddenly goes off because the oxygen
is about to run out. Mitch sends Jonse to connect the new tank while he starts doing CPR for Engel,
but there’s no response. Assuming it’s too late, Mitch pulls back and begins grieving with Jones,
who almost starts crying before he notices a slight movement from Engel. As the new tank begins
sending oxygen into the bell, the flashbacks come back to Engel’s mind and his friends sit
him up to offer him a mask and finally bring him back into consciousness. Now they have two hours
left of air. When the navy frigate contacts them again, they inform them that they’ll be in the
area of their coordinates in an hour and a half, but they still need to locate the exact position
because they don’t have maps since the oil company has closed all communications. Mitch points out
they should find the signal from the beacon, but the ship explains the sonar isn’t picking it up,
probably because the ship’s wreckage is scattered all over the place. The navy frigate promises to
speed up their search before they hang up and Jones says they should make the beacon float so
it appears on the sonar. It’s risky but if they don’t try the beacon will get caught in the deep
water currents. Mitch volunteers to go and while he’s getting ready, he gives his letter to Engel
so he can give it to his family in case something happens to him. He also shares the discussion he
had with his family about leaving his job, and in return, Engel explains he lost his wife in a car
crash. Engel still feels guilty about it because he thinks he didn’t pull hard enough to get her
out of the wreckage. Afterward, Mitch grabs the beacon and begins swimming up, going as far as the
umbilical lets him. Then he has to use a switch Jones made to send the beacon up,
but the beacon keeps slipping away, so Mitch has to take his gloves off. At that moment, a bunch of
jellyfish shows up, and they take advantage of his bare hands to start stinging him over and over,
making him lose the beacon. Mitch screams in pain but he still struggles against the little
beasts and manages to recover the beacon. Then he finally presses the button and lets it float
up while the jellyfish surrounds him and stings him to death. In the bell, Jones cries when they
stop hearing Mitch on the comms. Engel is furious at another loss and tells Jones to cut the gas,
but Jones is having a breakdown because he thinks he won’t see his wife ever again. Engel has to cut
the gas himself then he grabs Jones to shake him and make him calm down, but when he throws him to
the floor, Engel realizes he’s losing it too and lets the boy go. Nearby, Mitch’s body sinks to
the bottom of the ocean. Afterward, Jones takes out a knife and explains death by suffocation
is too slow. If rescuers don’t come in time, Engel should kill him to end things quickly,
because Jones doesn’t think he can do it himself. Before Engel can answer, another message from the
navy frigate comes. They’ve found the beacon, but it’ll take an hour for the divers to get there.
Seeing no other choice, Engel tells them he’ll make the bell float up before hanging up. The duo
disconnects the bell, which starts going up quite fast. However it suddenly comes to a stop when the
umbilical gets stuck in the ship’s wreckage. Engel points out they’ll have to swim up, but it’s going
to be tough because they must go up 170 feet, and Jones never did more than thirty. There’s also
only one suit left. Jones grabs his knife, ready to end things, but Engel announces Jones will be
the one going up with the suit while he stays to accelerate the decompression, pointing out that
it’s better for one of them to survive than none. Engel gives Mitch’s letter to Jones and gives him
instructions on how to survive the dive, but Jones can’t believe what’s happening and demands to
know why Engel is willing to do such a thing when he's the better swimmer. Engel confesses that on
the day of the car crash, there was a kid in the other car, and he failed to save him too. Now he
refuses to let another young person die. Accepting this, Jones puts on the suit and begins swimming
up while Engel gives him instructions through the comms. The bell is starting to fill up with water
but Engel ignores it, telling Jones to expect some pain because of the pressure change. Jones starts
screaming at the pain in his limbs, saying that he’s scared, but Jones encourages him through it.
Once Jones reaches the end of the umbilical, he takes a deep breath of air and detaches to
swim the last twenty feet without the suit. This causes decompression sickness and Jones starts
bleeding from the mouth, but he doesn’t give up and keeps going with his remaining strength until
he finally hits the surface. After floating for a few seconds, a boat sent by the navy finally
finds him and rescues him. The main ship tells Engel through the comms that Jones is fine. The
air in the bell finally runs out, and since the water is almost filling it, Engel takes out his
wife’s necklace and decides to try one last time. He swims out of the bell and starts going up,
however he can’t go far without a suit and he drowns. Sometime later, Jones reunites
with his wife, but it’s hard for him to let go of the trauma of what he went through.