In 1985, a crew is working hard in an oil rig called Sector 7. When one of the drills is having
trouble, a man goes underwater in a submarine and discovers the drill is stuck. In order to fix it,
he puts on underwater gear and leaves the submarine, which allows him to see a huge
and beautiful sea creature swimming by. At that moment, the sea floor cracks,
the ground starts shaking, and some mysterious little-glowing creatures begin swimming around
the man. He tries to touch one of them, but as soon as he stretches his hand, the pressure of
the water suddenly rises and starts pushing the man and the submarine upwards. The man holds onto
the drill and tries to use the radio to call for help, but now the little creatures are attacking
him and everything goes black. Years pass and it’s now 2011. The current crew in Sector 7 is having
a problem with the riser, so Hae-Joon instructs them to lock the valves while she controls the
pressure, during which she accidentally hurts her shoulder. With lots of coordinated effort, the
team holds onto the valves as the pipe is slowly lowered. This pipe almost hits a man in the head,
but they move away just in time and manage to fix the riser before it gets worse. Afterward, the
team discusses the fact they can’t seem to find oil. They consider Hae-Joon to take unnecessary
risks and be too impulsive, which triggers an argument. Hae-Joon refuses to give up and
swears she’ll find oil sooner or later. Meanwhile another employee scolds a young crewman and puts
him in crazy positions for almost dropping the pipe on his head earlier. The poor youngster is
scared and holds onto the pipe, which causes a weird liquid to come out of the platform. This
liquid also brings out a little glowing creature that the men don’t recognize. Back to Hae-Joon,
she’s at the clinic so Doctor Moon-Hyung can look at her injury. Dong-Soo comes to tease
Hae-Joon for this and tries to flirt, still not understanding that she isn’t interested.
The rest of the crew works on catching a bunch of those little sea creatures and puts them
into an aquarium to study. Marine biologist Hyun-Jung explains that the creatures are from
a chemosynthetic ecosystem, which exists under ours, but she doesn’t understand how they found
their way into Sector 7. While she’s distracted talking to the others, Chi-Soon reaches inside
the aquarium and retrieves one of the creatures, however the little animal immediately puts out
its tongue and hits Chi-Soon on the face. Moments later, Chi-Soon has a huge wound on his lips. When
Hyun-Jung is taking the elevator, Chi-Soon stops the doors and tries to give her a gift to confess
his feelings for her, but she turns him down and forces him out of the elevator. Sometime later,
Captain Hwang announces they’ve received orders to leave the rig. Since they can’t find oil, the
operation has been canceled. Hae-Joon gets angry, saying that they shouldn’t give up that easily,
then stomps out of the room refusing to give up. Dong-Soo goes after her and tries to comfort her,
but he only makes things worse. A few hours later, a helicopter drops Jung-Man in Sector
7. He used to work here when he was younger and he’s Hae-Joon’s uncle. Hae-Joon meets him with a
hug and everyone else has great respect for him because of his reputation in the industry. They
prepare a special meal to welcome him properly, and suddenly everyone is showing off their scars.
Hwang disapproves, saying scars are only signs of carelessness, but Hae-Joon shows her own scar
and says that the value of scars isn’t something that their captain could understand. Afterward,
Hae-Joon looks at her family's old photograph, and it’s revealed her dad was the guy that died
underwater years ago. Jung-Man approaches her to comfort her and Hae-Joon reminds him that
she wants to keep on digging to find her dad. With this in mind, Jung-Man talks to Hwang and
announces he will talk to the board to get a few more months of work. He’ll let Hwang take
all the credit if they find oil, but he’ll take the blame if they fail. The next day,
Jung-Man gives a motivational speech to the crew and everyone leaves their handprints on the pipes
for good luck. For three months, the crew works extremely hard, and they finally manage to find
oil. Now that the main problem is solved, they can catch a break. Jung-Man likes to shoot plates to
relax and keep his aim trained. Meanwhile Hae-Joon and Dong-Soo have a race on their bikes. They ride
around the rig at full speed, and Hae-Joon takes a shortcut through a staircase to make
a big jump. However when she’s about to reach the end, Dong-Soo manages to get there first by making
a huge jump that impresses Hae-Joon. Dong-Soo reminds her that if he won she had to kiss him,
but she’s cheeky and barely pecks his face instead of giving him a proper kiss. Afterward,
Hae-Joon and one of the crewmembers put on their gear and dive into the sea to check the structure
underneath the rig. When they check the pipe, they notice damage on the metal and begin working
on fixing it. Up in the rig, a strange presence begins sneaking around and hits the rope that is
holding Hae-Joon's crewmate, which begins pulling him in. The crew notices this and tries to pull
up, but instead the rope breaks and the man dies while falling to the bottom of the sea. At that
moment Hae-Joon’s tools malfunction and knock her out, so the crew has to pull her out of the sea
using the other rope. Suddenly she sees herself surrounded by weird-looking jellyfish and among
them she finds the dead face of her friend, but this turns out to be a dream and Hae-Joon wakes
up in the clinic. She has a breakdown because of grief, and Jung-Man comforts her before sending
her to her room to rest. Hyun-Jung visits her and wants to tell her something very important,
but before she can say anything, she’s interrupted by the others and leaves. Then Hyun-Jung goes
outside to do some thinking and is approached by Chi-Soon, who keeps bothering her with his
advances. This is seen by the working crew. Later in the evening, Hae-Joon sits at her friend’s
machinery to grieve for him, but she’s suddenly startled when the body of Hyun-Jung falls on the
window. She’s taken to the clinic and Moon-Hyung inspects the body, deciding that Hyun-Jung ended
things for herself. Hae-Joon refuses to believe it because Hyung-Jung wasn’t in that state of mind,
she also thinks the doctor is rushing the investigation. At that moment, Moon-Hyung notices
something else: there’s male seed on the body, proving someone may’ve pushed her after all. The
crew immediately gets suspicious of Chi-Soon and two crewmembers go to find him. Chi-Soon swears he
didn’t do anything, but the others beat him up to capture him. At the clinic, Moon-Hyung is looking
at the seed under the microscope and discovers there’s something different about it. At that
moment the guys come with Chi-Soon to show him the body, and Chi-Soon immediately starts crying. To
be sure of the evidence, Moon-Hyung asks them to get hair from Chi-Soon for a DNA test. Meanwhile
Hae-Joon remembers Hyun-Jung tried to tell her something and guesses something is wrong,
so she goes to inspect the laboratory. She hears a noise coming from a cabinet and when she opens it,
she’s startled by a creature, but it’s just a rat. Then she looks outside the laboratory and
discovers that substance that looks like male-seed is leaking out of the pipes. In the clinic,
Moon-Hyung is doing the DNA test when he suddenly hears something. He checks the door
and sees nothing, but someone off-screen hits him and kills him in seconds. Back to Hae-Joon,
she continues to look around and finds the broken rope, but at that moment she hears someone running
away. When she goes to check, she finds Hwang, who tells her Moon-Hyung is dead. The crew gets
together to discuss theories and discovers both bodies are gone. The guys think Chi-Soon did it,
but Jung-Man is skeptical. Meanwhile Chi-Soon is tied up in the basement and begins shaking
in fear when he sees a strange shadow approaching him. At that moment the crew arrives and finds a
trail of blood and that seed-like substance that they follow to find Chi-Soon hurt in a corner.
At first they think he’s dead, but when they come closer, Chi-Soon wakes up and starts to
freak out as he announces there’s a monster in the building. Suddenly the entire rig starts to
shake and Chi-Soon takes the chance to run away. The crew splits into two groups to go after him,
and when Hae-Joon and Dong-Soo go down a corridor, they’re shocked to discover a huge monster walking
on the ceiling. In another corridor, Chi-Soon is trying to escape without being noticed,
but the monster finds him and immediately kills him. Moments later, Hae-Joon and Dong-Soo make
it to the generator room and discover the monster made a nest there. The whole place is now covered
in goo and has the dead crew bodies in it, which has turned off the power. Afterward, the
crew gathers in the comms room and Hae-Joon tells them the monster is real. They try to make a call
for help but communicators are down, so Jung-Man grabs his weapon and announces he’ll be fighting
back. Hae-Joon, Dong-Soo, and Hwang decide to go with him, and they start making their way down
the corridors to find the monster. Instead they end up finding another two crewmen, and the noise
they make gets the attention of the terrifying creature, who shows up behind the duo. Hae-Joon
and Jung-Man want to shoot, but they don’t have a clear aim. Jung-Man asks the guy to move the
clutter, but he misunderstands and begins clapping instead. At that moment the creature reveals its
long tongue and catches the crewmember to begin tossing him around, hurting him by beating him
against walls and clutter. Hae-Joon and Jung-Man take the opening and come closer to shoot at the
monster, allowing the two crewmembers to escape. The monster falls to the ground for a few seconds,
but it turns out it’s only taking a moment to recover from the bullet wounds and now it’s ready
to attack again. Then Jung-Man takes out a lighter and starts a flame before throwing it at the
monster, who immediately catches on fire. Furious, it begins chasing the crew, who runs away and
tries to close a door behind them, but the monster easily pushes it down. Next the crew gets inside
the elevator, and the monster crashes against the door, unable to break this one. However it
finds the window and it breaks it to make its long tongue come through. The crew dodges every attack,
and when Hae-Joon sees an opening, she cuts the tongue down with an ax. Afterward the crew goes
to the lab and takes care of their wounds while Captain Hwang gets suspicious of Jung-Man, saying
he volunteered to come back to this lousy rig and that he knew to use fire against the creature.
Jung-Man responds by breaking one of the aquarium boxes and grabbing one of the little creatures to
set it on fire, saying it’ll keep the flame up for 24 hours. Then he shares the whole story:
when Hae-Joon’s father found the creatures years ago, the science team immediately noticed that the
seed-like liquid these animals produce could be used as a much better oil. However there was an
ethical argument because they would be burning living beings. The team started to breed and
experiment with these creatures, and one of them ended up growing into the giant monster they met
earlier. Hun-Jung discovered the truth while she was doing her research, and when she threatened
to tell the others, Jung-Man killed her. Back to the present, an angry Hae-Joon feels betrayed by
her own uncle, but Jung-Man says he’s come back to pay for his mistakes. Suddenly they begin
hearing noises and the monster appears at the window, which it quickly breaks to get inside
and attack. The crew begins running away and Jung-Man gets captured, but he tells Hae-Joon
to leave him and escape. As Jung-Man is tossed around by that sticky tongue, he manages to grab
a bottle of burning medicine and throws it at the monster, who immediately hisses in pain and
drops the man before running from the room. The crew goes looking for their weapons and Hae-Joon
volunteers to go to the generator room to restore the power. Dong-Soo goes with her and watches
in shock how Hae-Joon isn’t afraid to reach into the goo and pull the lever to finally bring power
back. Meanwhile Hwang and the two young guys go down a corridor and are startled when they
suddenly find the monster underneath their feet. They try to hold him down, but the creature is too
strong and bursts through the floor to attack. The men try to shoot it to no avail and the creature
begins chasing them. As they run, one of the young men trips and his friend goes back to help him,
but Hwang keeps running and enters another room where he locks the door, not caring about the
pleas of his terrified men. The friends try to cling to each other, but the monster uses his
tongue to pull them apart and one by one, he kills the two guys without mercy. By the time Hae-Joon
and Dong-Soo come back, they’re shocked to see nobody around. It turns out Hwang has selfishly
taken the submarine alone and is trying to escape by going underwater. However he barely manages to
travel for a few minutes before the body of one of his crewmembers hits the window, and the creature
suddenly appears on top of the submarine to break all the glass and let the water into the vehicle,
causing Hwang to drown. Back on the rig, Jung-Man initiates the building’s self-destruction sequence
before going outside to shoot a flare. Hae-Joon and Dong-Soo want to stop it, but only the
captain has the power to do so. At that moment the monster comes back and Jung-Man meets him
with a flamethrower, which only burns the monster a little bit and doesn’t have much of an impact.
Next Jung-Man begins fighting the monster with a pipe, and after exchanging a few hits and being
tossed around, he stabs the creature and holds onto the metal as a trap. Now Jung-Man can use
the lighter again and start a better fire, which begins burning the creature at great speed.
However Jung-Man got covered in the weird liquid during the fight and he ends up dying in the fire.
The creature begins shaking in pain and looking for water, so Dong-Soo has to push Hae-Joon out
of its way. Unfortunately the monster uses the chance to grab Dong-Soo as well, and it brings
him with it when it jumps into the ocean together with Jung-Man’s body. Devastated, Hae-Joon falls
to her knees and suddenly hears something: it looks like Dong-Soo managed to save himself by
holding onto a rope. Hae-Joon immediately gets her bike and uses it to pull at the rope. The monster
jumps out of the water to try to attack, but Hae-Joon pulls up Dong-Soo just in time. Suddenly
the bike slips and Hae-joon falls, hurting her shoulder in the process, but luckily Dong-Soo
can climb the rest of the way up. With one minute left for self-destruction, Hae-Joon is happy to
see Dong-Soo again, but the moment is ruined when the monster crawls back. Dong-Soo grabs it by his
mouth and begins struggling against it, wanting to give Hae-Joon the chance to escape. The monster
doesn’t give up and throws Dong-Soo in the air before jumping after him and killing him with
a quick bite. The creature doesn’t stay in the water though, it quickly begins climbing back,
so Hae-Joon gets on her bike and drives near the monster to get its attention. Using a gun,
she shoots at it to put it near the edge to delay it, then she drives away to make a huge jump
and turns around a corner to go under a narrow pathway. When the monster tries to follow her,
it gets stuck in the structure, so Hae-Joon pulls another quick maneuver and throws the bike at the
creature, causing a huge explosion. Unfortunately, the monster is thoroughly burned yet still not
dead. Hae-Joon runs to the drilling area and hides in the cabin, where she carefully presses a button
to open a door from afar. The monster goes into the other room thinking someone is there,
and then Hae-Joon closes the door, thinking she’s managed to lock it up. However the monster still
escapes and comes back through the ceiling, then it begins chasing Hae-Joon through the pipes.
Hae-Joon grabs a wrench and begins hitting the monster, only to get pushed against the machinery.
Then Hae-Joon finds some rope and lets the crane take her up. The monster begins climbing up too,
so Hae-Joon jumps onto a pipe and causes the monster to fall right into the drilling spot.
She falls as well and hurts her leg, but at least she dodges the pipe just in time. Next, Hae-Joon
rushes to grab the remote control and stands still to lure the monster in. Once it’s close enough,
she presses the button, and the drill comes down, holding the monster captive. However its limbs are
still free and manage to grab Hae-Joon’s leg, making her fall. Hae-Joon struggles against
the hold and drags her body on the floor until she reaches the control and activates the drill,
which begins slashing through the monster’s body and finally kills it. Afterward,
the rig begins to self-destruct. Hae-Joon doesn’t let all the explosions scare her
and she gets on her bike to jump out of the building and reach a nearby platform,
where she shoots another flare. Soon a helicopter comes by and rescues her. A few years later, an
older Hae-Joon comes back to Sector 7 and checks the underwater pipes, where she begins crying when
she discovers all the encouraging messages her old crew had written on them before the tragedy.