During the Cold War in Romania, a group of looters enters a forbidden area, where a guide
indicates they must climb Carpathian Mountains to reach their destination. After lots of effort,
they find an abandoned church with a special passage that takes them to the crypts. Here, the
men find a beautiful engraved art showing weird winged demons. They've heard of a secret cave
full of gold yet they can't find an entrance, so they use explosives to open a hole in the ground,
which destroys another beautiful yet creepy piece of art. Unfortunately soon the entire church
starts shaking and the floor collapses, causing the men to fall underground and a landslide to
demolish the building, keeping the group buried underneath. At least they've all survived,
thus they decide to check out the weird bat-like sound they keep hearing. Thirty years later,
a group of scientists begin an excavation on the site the church used to be. They find the cave and
notice there's a river that extends deep inside it, so they decide to hire a group of expert
explorers to investigate. Meanwhile in Mexico, Tyler and his team of divers are looking for an
underwater cave in Yucatan. They don't have much luck and most of the team gives up after a while,
but Tyler stays until he finds it. He's rather smug when he returns to the boat,
but his brother Jack scolds him because it's important for his safety to stop when the group
calls time. Later in the evening, they get a call from Doctor Nikolai, who wants to hire them to
explore the cave under the church ruins, promising it goes deeper than nineteen miles and he wants
to record it all. The team finds the concept intriguing and accepts the job. The following day,
the team arrives at the exploration site and while they get their equipment ready,
Tyler notices Catherine, one of the scientists. He immediately goes to introduce himself with the
intention of flirting, but he misses his chance because the cameramen Alex is there too. At least
Tyler gets to show off his diving equipment, but their talk is interrupted when Nikolai introduces
Jack to Catherine, and the two of them immediately hit it off because of their shared interest in
cave ecosystems. A jealous Tyler is left behind and he notices Alex taking pictures of something
weird which turn out to be the rests of a human being that they found in the cave. In the evening,
the teams of scientists and explorers meet up to go over the information they have so far. There's
an enormous cavern about 300 feet down, which they have named the "Titan Hall", with a sump
that leads to an underwater passageway. They need to find a dry cavern somewhere
in the system where they can set up camp and the scientists can gather samples. Since it's so big,
they'll be down there for twelve days. Survival expert Top reminds everyone that they've already
found a body, so they need to respect the cave and be careful. After the meeting, Nikolai tells
Tyler and Catherine the story behind the cave. It's said that the church was built to seal the
cave as a display of God's protective power. From what he can gather from the broken art,
the knights entered this cave to battle demons. The next morning, the team begins making their way
down the hole. The Titan Hall is absolutely beautiful, and the group immediately begins
preparing their equipment for the exploration. They decide Briggs shall be the first scout, and
he gladly jumps into the water, finding a curve up ahead after some minutes of swimming. Using his
communicator, he lets the others know he may lose his connection soon and that'll call them when
he can set up the fiber optics on the other side. While the group waits, Tyler expresses how unhappy
he is that he wasn't chosen because he thinks he's the best scout on the team, but Jack sticks to his
decision as a way to punish Tyler for what he did in Mexico. While Briggs continues his exploration,
Catherine approaches Tyler and gives him a spider in a tube as a "consolation prize" and they bond
over the fact they both want to be the first ones to discover something nobody has ever seen before.
The rest of the team is starting to wonder if they need to send someone after Briggs because
he's taking too long, but at that moment Briggs calls and informs them he's found the perfect
place to set up camp at 2.4 miles from the main cave, so he sends them the scan he took of the
route with the sonar. Suddenly Briggs is startled when something runs past him, and he picks it up
to show to the team some kind of weird-looking mole. The animal protests the hold and runs away,
but then Briggs goes very pale and says there's something else there. The connection starts
failing and after a few seconds they lose contact with Briggs. The team blames it on signal issues
and decides to send Tyler and sonar expert Strode to fix the cables while the others explore the
tunnel. They're amazed by all they see as they swim, including a giant animal skeleton. While
most of the team stays to study it, Jack and Top reach the base and find Briggs' equipment but not
the man himself. They call out for him and Briggs responds from a small opening in the cavern wall,
where he's found old equipment that proves they aren't the first humans that visited this place.
Meanwhile Tyler and Strode are working on the cables underwater when they hear a strange noise.
Strode thinks he saw one of the moles and goes to investigate, finding a new cavern and the mole
brutally dead on the ground. At the same time that Tyler discovers the cables look chewed,
Strode suddenly is attacked by a demonic-looking beast. Tyler can hear his screaming through the
communicator and tries looking for him as the rest of the teams joins Jack and Top. Strode
struggles against the creature's hold and his rebreather floats away until it crashes against
the cave's walls, instantly exploding and causing a collapse that blocks the entrance. The shockwave
from the explosion pushes Tyler to the surface, and he quickly tells the others what happened.
Jack and Top go to investigate, only to find the entrance blocked and rocks still coming down, so
they have to quickly swim back. At the base, Jack informs the team that they're trapped and nobody
will consider them missing until the planned twelve days pass. They need to find a way out,
but Briggs is nervous about the idea because obviously something attacked Strode. However Tyler
tells him Strode probably had a hallucination, and Top agrees their priority must be to survive.
Nikolai thinks the cave is too big to explore and they would get lost, therefore the smartest
thing would be to wait out those twelve days, but Jack doesn't think any rescuer in the world would
find them because the best ones are down here. Afterward, Jack and Top go to look for an exit
while the others rest. Briggs is angry with Tyler for how he downplayed Strode's death and makes
mean implications about their splitting, so Tyler punches him. A fistfight ensues that Charlie has
to stop as she reminds them to behave. Catherine uses her time to analyze samples she's taken
from the dead mole and a salamander, and when he shows them to Nikolai, he confirms her suspicions:
there's a strange parasite in the cave. Moments later, Jack and Top find an area with bubbling
water and signs more people were here before. Then they crawl through a very narrow tunnel,
only to find their way blocked by a bunch of white scorpions. Jack turns on a flare for a better look
and ends up attacked by a strange beast that tries to take him away, but Jack manages to cut off the
beast's claw to free himself. Noticing Jack's back has been severely hurt, Top immediately takes him
back to the base to tend his wounds. Catherine takes a look at the claw and theorizes all the
animals in the cave used to be the same ones as the surface, but after a long time living
down here, they evolved and lost their sight in exchange for a heightened sense of smell and
hearing. Nikolai thinks they've found a whole new ecosystem, and Alex points out the claw looks like
the demons on the church's art. The scientists also note that the claw has the same parasite
inside it. Even with a dangerous creature out there, Nikolai thinks they should wait, but the
team agrees they should move on and find an exit before they're eaten. As they explore the cave,
they find a bunch of human bones with teeth marks on them, confirming the beast is a predator. Then
they cross the narrow tunnel and reach a fork in the path, so Jack goes to scout ahead. This annoys
Briggs because they could just use a sonar, but when he checks the path Jack didn't take,
the sonar indicates something moved. Scared of what may be inside, Briggs eats his pride and
accepts to follow Jack with the others. The group begins moving upward by climbing a few walls,
and Jack shocks Top by saying he can smell water. A moment later, Jack's proven right when they find
a river. He throws a flare in it to check for any beasts, and Alex records the whole process,
which makes Jack uncharacteristically angry. He pushes Alex away and drops the camera in the
water before Alex calls him out for it, the words make Jack snap out of his anger and he apologizes.
Afterward Jack announces they'll rest for twenty minutes and goes to sit alone on a rock as his
body shakes in pain. Top sees this from afar and tells Tyler the medicine isn't working. To Jack's
sudden shock, he can hear what everyone is saying even if they aren't near him. At that moment,
Tyler comes to check on his brother, but Jack swears he's fine and asks him to trust him.
Once they're done resting, the group returns to the water while Jack continues to hide the pain
he keeps feeling. There's a strong current here that keeps on pushing them, and the team worries
the rapids could make them lost. An angry Jack argues this is the only available exit so the
team has no choice but to follow him, even when Alex points out that Jack has a death wish. As the
rapids take the team away, Catherine gets stuck and Tyler can't help her because the current is
in the way. The water pushes Tyler away from the group while Nikolai hangs onto a rock because he
badly hurt his leg when the current pushed him against the cave wall. Eventually Tyler ends
up falling down a waterfall, and Catherine soon follows him, explaining to Tyler that something
had caught her back there. Then Charlie catches up to them and informs them there's something
weird in the water. When Tyler checks, he discovers some weird eels with big teeth.
He uses a flare to scare them away, and at that moment they are contacted by the other three guys,
who also light a flare to mark their location. As Tyler and the girls swim towards the others,
they hear Nikolai fall off the waterfall too. Jack decides to go looking for him but at that moment
they hear Nikolai screaming because something is attacking him. Catherine wants to help him,
but Top reminds the group to stay in a circle for protection and wait patiently. Jack follows
the trail of blood and discovers a huge horrific creature taking Nikolai away, so he follows them.
When he resurfaces, he sees the creature taking a dead Nikolai through a very narrow passage and
notices tattooed letters on the creature's hand, meaning this beast used to be human. Back to the
group, Alex feels something moving in the water and starts kicking like crazy, which seems to be
enough to scare whatever it was away. Jack shows up and tells them about what happened, causing
Catherine to blame him for Nikolai's death because he never wanted to leave the camp. The group keeps
on moving and they hear weird sounds that they identify as echolocation, proving the theory of
the creatures being almost sightless. Eventually they find a current that seems to be the main
water flow, which should get them outside, but they aren't sure if it's safe considering the
creatures. Jack decides they should start climbing the walls and Charlie volunteers to go first,
but Jack turns her down, saying he and Tyler have this. As they get the equipment ready, Catherine
tells Tyler that Jack is too sick to lead, which is overhead by Jack. Charlie takes advantage of
their being distracted to start climbing first, moving expertly quickly to reach an edge in a
matter of minutes. At the top, Charlie can feel a draught, and the others get ready to follow her.
However when Charlie looks deeper inside, she finds a giant bat-like creature that roars at
her. Charlie immediately backs away and slips off the edge, causing her to fall and be unconscious
for a few seconds. Thankfully the rope holds her up and when she wakes up, she discovers the beast
is coming after her. The team begins working on getting her down but they aren't fast enough,
so Charlie begins swimming her rope as she uses her flamethrower to keep the beast at bay. Once
she has enough momentum, Charlie cuts the rope and jumps on the adjacent wall, thinking she's finally
safe. Unfortunately the creature can fly and immediately jumps on her to start attacking her,
thus Charlie tries to defend herself and uses her flamethrower to light the beast on fire.
The creature falls dead, but sadly Charlie dies as well. After the team retrieves Charlie's body,
they notice Jack is looking very different: his pupils are slated and his skin is pale.
Catherine finally realizes the parasite entered Jack's body through his wounds and he's changing
like the other animals and people that were here before. Tyler tries to defend his brother and his
decisions, but Catherine, Alex, and Briggs decide not to listen to him anymore and take the main
water flow they saw earlier. Then Jack, Tyler, and Top climb up the wall since the corridor is safe
now that Charlie killed the creature. Soon they reach a steep frozen cliff, and Jack decides they
need to go down. Tyler slips and barely manages to land on an edge, but his rebreather falls and hits
Top, making him slide down and hurt his leg. The rebreather slides down as well and makes
a hole in an ice wall, revealing another cavern. The trio investigates the area and finds lots of
skeletons in medieval armor, confirming Nikolai's story. Deeper inside, Jack finds daylight under
the gurgling water, meaning that's their way out. Suddenly Jack begins feeling sick again,
but he ignores it and tells their friends to leave while he searches for the others. Tyler refuses to
accept this plan because Jack is clearly sick, which makes Jack so angry he almost starts a
fight. Top immediately stops him and convinces him that Tyler is the one that should look for
the others. Meanwhile Catherine and the others are getting ready to swim, but Briggs hears a
creature approaching. He tells his friends to run away while he lights a flare, but the beast isn't
scared and attacks him anyway. At the same time, Jack and Top are found by a creature
too. They move out of the way before they can get hurt, but when the beast flies by it takes one of
their rebreathers. Back to Tyler, he makes his way through the water while ignoring the eels and when
he resurfaces, he finds his team's equipment covered in blood. Then he hears screaming and
follows it to find Briggs being taken away by the beast. Tyler crawls through the crevice to
try to save him but it's too late, the monster has impaled Briggs on the stalactites. Before dying,
Briggs tells Tyler that Catherine is deeper in the tunnel and gives him his pickaxe. At that moment
the monster shows up and Tyler begins running away through the tunnel only to find another creature
on the other side. Thankfully he also bumps into Catherine, who uses the sonar to send strong
sound waves that scare the creature away because sound is their weakness. The duo escapes together
until they find a river, and they jump into it to swim away as they share Tyler's rebreather. They
have to go through a narrow passage that causes them to lose the rebreather and Catherine loses
consciousness, so Tyler has two drag her along to the surface. Thanks to Tyler's use of CPR,
Catherine quickly wakes up. At that moment they hear a weird sound, but it turns out to be Alex,
who warns them the monster is coming after him. Tyler finds an ice vein and assumes it connects
to the one Jack found earlier, so the trio immediately begins climbing. Moments later, they
find the cavern with the skeletons and reunite with Jack and Top. The creatures are hanging
from the ceiling as if waiting for something, meaning they need to hurry up. They don't have
enough rebreathers even if they share, so Jack decides he'll recover the one the monster stole.
The rest of the group moves towards the water only to slip and drop a rebreather, which makes
enough noise to catch a creature's attention. Alex uses the sonar to keep the creature at bay but it
starts breaking stalactites that fall on Alex and hurt his leg. Tyler tries to go back to help him
but the monster gets there first and kills Alex. Another creature approaches them from the water,
so Top and Catherine try hiding to avoid its echolocation. The monster finds them anyway
and it won't leave even when Top hits it with the rebreather, but Tyler jumps in and kills it
with a machete. Meanwhile Jack runs away from a third monster and climbs a wall to retrieve the
last rebreather, which he reaches after hitting the creature to make it fall. More creatures
are coming, so Jack tosses one of the tanks to his brother and puts a flare in the other to then toss
it in the water. The tank explodes and the whole cave begins shaking, distracting all the monsters
except for one. That creature flies toward Jack, who jumps on top of it and brings it down into the
water as a sacrifice to help his friends. Before the cave finishes crumbling, Tyler, Catherine,
and Top swim towards the light and manage to finally safely reach the outside. Some days later
in the city, Tyler says goodbye to Top before meeting with Catherine at a café. He wonders
if Jack could've survived outside considering his mutation, but Catherine isn't sure anymore. As she
kisses Tyler on the cheek, she explains she thinks the parasite wanted to come out, and Tyler notices
her pupils are changing like Jack's did. Then Catherine goes away, and a terrified Tyler tries
to chase her to stop the parasite from taking over humanity, but he loses her in the crowd.