In a forest, Drsa wakes up when she hears a plane arrive. She gets her weapon ready and as soon as a man gets off the plane, he gets shot in the head. Meanwhile former Marine Levi wakes up from a nightmare and decides to clear his head by resting at the beach.
In the morning, he receives a text telling him to go to the Marine Corps Base. Later at the base, Levi meets Agent Bartholomew. She knows he has an outstanding career as an elite sniper and no family or friends, so she's here to offer him a job.
In Lithuania, Drsa meets with her father at the cemetery for the anniversary of her mother’s death. Her dad recently learned that it's too late to treat his cancer, so he announces that if he isn’t dead by February 14th, he’ll self-delete so he can finally reunite with his wife on Valentine’s Day. In September, Levi wakes up in a plane from an anesthesia-induced sleep.
He asks a soldier where they are, but she isn’t authorized to say. She takes all his personal items and tells him they’re reaching a no-fly zone. He’ll have to jump now and walk the rest of the way to his destination.
Levi grabs the assigned bag and jumps, waiting for the right moment to open the parachute. He lands safely and begins his long journey, which entails lots of climbing, trekking, and camping. Eventually Levi reaches the edge of a huge gorge.
Here he meets soldier J. D. , who has been here for a year and doesn’t know where they are either.
The men go to the West Tower, which is completely self-sufficient with solar energy, a backup generator, a rain system, a vegetable garden, and plenty of wild game to hunt. On the other side of the gorge there’s the East Tower, but contact with that guard is forbidden. The gorge has land mines and tactical barriers on the north and south, but there’s no way to go across.
As J. D. continues to show Levi around, he explains the mission.
Every day Levi must walk the western rim and restock the ammo in the automatic gun turrets, which are located every 600 meters. He must also check the containment fence, suspended mines, and cloakers, which are devices that send false signals to all satellites to keep the gorge invisible. The radio checks in every thirty days, but there’s no way to communicate with the outside.
In case of an emergency there’s a panic button that will activate the Stray Dog Protocol, but J. D. doesn’t know what that entails and advices Levi to run if it ever happens.
J. D. explains that this place is as old as the Cold War and back then, military teams from different nations made a pact to keep the place secret, so not even heads of state and presidents know about it.
Levi’s mission is simply to keep things in the gorge from coming out. It sounds crazy and J. D.
used to think so too until he saw something that the guards have named “Hollow Men”, which can now be heard screeching in the gorge. J. D.
says that in the late 1940s, three horseback battalions were sent into the gorge to clean it out but nobody returned, so the military is concentrating on doing containment instead. Afterward J. D.
leaves the tower and hikes all the way to the pick-up point. A helicopter picks him up with a rope and a soldier asks for his identity. Once J.
D. says the password, the soldier shoots him in the head and drops his body in the mountains. A month later Levi has gotten used to his very boring routine.
He walks the perimeter, does all the maintenance on the security systems, and does his check-in on the radio. The voice on the other side only wants to know if everything is fine and hangs up before Levi can ask any questions. While working on the garden, Levi notices some movement on the other tower.
He rushes to his binoculars and sees Drsa cutting wood. She sees him too but ignores him. One night during a storm, Levi is startled awake by a nightmare and breaks a glass, which makes him think he’s being attacked so he grabs a gun.
When he feels pain in his foot, he realizes he stepped on the glass and it was only an accident, so he calms down. While cleaning his foot, he notices that Drsa is still awake and watching him, however she goes to bed when she notices him watching her back. Another month passes and Drsa has to celebrate her birthday alone.
Tired of the boredom, she shoots a gun to get Levi’s attention and holds up a sign asking for his name. Levi writes back that they aren’t supposed to talk, so Drsa responds with her name and saying she’ll do whatever she wants on her birthday. An amused Levi replies with his name too and raises a glass so they can make a toast.
Next Drsa plays music really loudly and while she dances, he asks Levi to dance along. He explains he is a terrible dancer but an amazing sniper, so Drsa responds by shooting his water bottle. Levi quickly grabs his gun and shoots her bottle too, then Drsa puts another bottle on her head intending to continue the game.
However she stops everything when she notices something moving in the gorge. When Levi looks down, he discovers strange creatures that look like people made of trees climbing up his side of the gorge wall. He immediately shoots one of these monsters and causes it to fall onto a mine, which blows up and brings most of the climbers down.
There still are a few creatures climbing, so Levi continues shooting and Drsa helps him using her machine gun. By working together they bring down all the creatures in a matter of minutes. When they think it’s over, a creature reaches Drsa’s balcony so she shoots it with her gun a few times before kicking it off.
Then the monsters begin climbing the wall on her side and both soldiers have to work together again to stop them. Another creature falls on a mine and a new explosion happens, causing a piece of metal to fly and hurt Drsa in the shoulder. Then the creatures start climbing both walls at the same time, so Levi and Drsa use the machine guns to kill them and activate more mines as backup, this time effectively killing the monsters until no more show up.
In the morning, Drsa takes care of her wounds and tells Levi it was her best birthday ever. Then he goes down to fix all the damage and replace the mines. Suddenly a drone flies out of the gorge and flies off.
In December Levi puts up a bunch of lights around a tree in the forest to simulate a Christmas tree, he also puts up a sign for Drsa. From then on they start sharing lots of activities together, like making snowmen with each other’s names, playing chess, and playing drums made of pots. On February 14th, Levi sees Drsa crying in front of a family picture.
He asks why she didn’t go on patrol earlier and she tells him she’s been having a hard day, so she wishes he could be there with her. Using all the tools from the armory, Levi builds a special contraption and shoots a rocket across the gorge, which gets stuck on a tree. It also starts a fire but Drsa runs to put it off and notices there’s a rope attached to the rocket.
She ties the rope firmly to the tree before running back to the tower, where she sees Levi asking her to dinner. Then Levi slides down the rope to cross the gorge, only to get stuck mid-way. Luckily he’s wearing gloves so he uses his hands to grab the rope and keep on moving.
Soon he makes it to Drsa’s tower and gives her flowers. She points out he stinks because of all the exercise he just did, so Levi agrees to take a shower. While he’s washing, Drsa takes his uniform and leaves some nice clothes from a previous occupant.
During dinner they discuss their experiences as mercenaries and what this kind of job does to their minds. Then they share a dance and end up doing the dirty. In the morning Levi begins sliding on the rope to return to his tower.
At that moment the Hollow Men appear on his wall and hit a mine, causing a huge explosion that cuts the rope. Levi begins falling and immediately opens his parachute while Drsa rushes to her armory to grab a bunch of weapons and a parachute of her own. She jumps into the gorge and after a few miles of falling, she lands right into a river.
On the surface, her parachute gets stuck on a rock, so Drsa has to cut it off. The current immediately begins pushing her and her body hits a rock, causing her to accidentally drop the auto-ascender. The river is still pushing Drsa, so she grabs onto some tree roots to reach land.
Meanwhile Levi is unconscious and hanging off a tree. Suddenly a branch starts moving, revealing it’s a monster. When it gets close to Levi, he wakes up and begins shooting at it while untangling the ropes from the tree.
He falls to the ground and lands on what appears to be mud, but the adhesive substance begins moving because it’s actually the mouth of another monster. The substance traps Levi as the mouth slowly starts to close. Thankfully Drsa heard the gunshots and arrives to open fire at the creature, rescuing Levi just in time.
After Drsa gets the substance off Levi’s body with a knife, the duo starts making their way to the wall to find the rope. Suddenly they hear screeching before two Hollow Men appear on horses that also look like trees. The riders keep attacking Levi and Drsa with swords and one of them even jumps off the horse to jump on Drsa.
She stabs it in the neck with a knife to make him fall and then shoots it in the face. The other rider takes Levi’s gun using a whip so he takes out his gun to shoot both the horse and the rider, killing them in seconds. Afterward Drsa checks her bag and discovers the auto-ascender fell in the river.
Since the walls are too high for them to climb, they’re stuck there. She thinks they should follow the river because it has to drain out somewhere. Levi takes a closer look at the creature he killed and discovers a very old military uniform.
This means Hollow Men are the lost horseback battalions from the story. The duo keeps moving through the forest, finding hundreds of skeletons on the way. Eventually they find an abandoned town where all the buildings are covered by mutated plants.
There’s also damage on the ground from an earthquake. They can hear screeching indicating more monsters are coming so the duo decides to hide inside a church, where they find skeletons on the seats. Levi can smell cyanide, which means these people self-deleted because they couldn’t stand living in this place anymore.
Suddenly they hear a noise and Drsa looks up to find a huge spider that jumps to attack her. She immediately pushes it off and falls to the ground, where she sees another spider coming. As she stabs it, two more spiders attack Levi who quickly shoots them.
Soon lots of spiders surround them and both soldiers keep on firing to kill as many as possible. Drsa runs out of bullets so Levi rescues her while she reloads. They start heading toward the door but at that moment some Hollow Men burst in.
Now the soldiers have to divide their attention between shooting the spiders and the Hollow Men. Drsa begins pounding a creature’s head on a table and throws it off, so the spiders start climbing on it too. Levi also has to fight a few creatures up close and after some struggle he manages to shoot them down.
Then Drsa shoots a grenade at the back wall to make a huge hole that allows them to escape. Levi and Drsa run through the forest but they can still hear creatures screeching in the distance. Since they’re too exposed here, they decide to go closer to the wall.
They find an old door and get inside to discover a facility that was overtaken by the plants too. Deeper inside they learn this used to be a military lab. There are skeletons and a generator that Drsa turns on.
Levi finds a tape so they play it on the projector. In the film, a scientist explains that it’s 1946 and that this community was formed as part of a top-secret coalition of nations. During the last years of World War II, they worked on creating biochemical missiles.
However six days before the recording of this film, an earthquake hit the area and caused a chemical leak. The scientists started to work around the clock to contain the toxin, which soon merged with the DNA of the local flora and fauna and mutated them. Being exposed to the toxin for only a few hours doesn’t do anything, but the more they worked the more exposed the scientists were and it was slowly affecting them too, as proven by the woman on the screen showing the branches on her stomach.
Thankfully since the base was in the gorge, the toxin never reached the rest of humanity. In another room, Levi and Drsa find a computer from the late 2000s, which means that after the initial incident people kept coming to continue the experiment. Levi turns on the computer and discovers this place belongs to Darklake, a private paramilitary corporation.
The drone they saw the other day was collecting samples for Darklake’s experiments, which are trying to create super-soldiers. The towers and the hired guards help them keep the secret. In the computer Levi also finds an explanation for the “Stray Dog” protocol, which turns out to be a nuclear bomb that will sterilize the area if a leak happens.
As the duo makes their way out, Levi tells Drsa that coming to this mission was worth it because he got to meet her. They open the door and throw a smoke grenade before coming out. Suddenly the Hollow Man’s whip grabs her ankle and drags her away.
As the rider takes her through the forest, Drsa tries cutting the whip with her knife but she accidentally crashes against a tree and drops it. She also tries shooting, but she can’t aim correctly when she keeps getting hit by rocks and logs. Levi follows the rider’s tracks and eventually ends up finding the facility with the nuclear bomb.
Inside, the Hollow Man has hung Drsa and turns the branch stuck in her stomach to torture her before taking it out. Drsa kicks him in the chest and untangles the ropes from the beam to free herself. Then she pushes the Hollow Man and grabs a piece of metal as a weapon.
The Hollow Man keeps on overpowering her and the snake in his chest tries to attack too, but Drsa never gives up and manages to decapitate the snake before throwing the metal at the Hollow Man. It doesn’t cause much damage but it angers the Hollow Man, who begins chasing her around the room as he attacks her with his sword. Drsa uses bones as a shield and makes a torch from the bonfire, which allows her to burn her enemy.
As the Hollow Man yells in pain, Levi finally arrives and shoots him to death. After the soldiers reunite with a hug, Levi checks the Hollow Man and notices the name in the uniform, recognizing it as the first guard that worked in his tower. After getting the creature’s sword, the duo starts making their way out and finds the place where the initial leak happened.
There are mutated creatures everywhere stuck in the goo created by the toxin. Levi and Drsa begin crossing the bridge while being creeped out by the faces breathing in the goo. Drsa accidentally steps on a root and when she pulls at the sticky substance, the bodies begin falling to try to capture her.
Levi uses the sword to quickly cut all the roots and rescue her. Soon bigger creatures are waking up and roaring at them. As Levi continues to open the way with the sword, a goo whip grabs his ankle and makes him fall.
Drsa shoots a grenade behind them and causes a big explosion. The creatures catch on fire and the flames spread through the entire goo structure while the duo reaches the stairs to finally get out. Afterward they search the town and find military jeeps in a garage.
They do some maintenance on an engine and take off in one of the jeeps, driving for hours until they find the broken rope. Levi immediately ties the rope to the jeep while Drsa shoots at the Hollow Men who try to approach them. When she runs out of bullets they just trade places.
Once the jeep is ready, they begin ascending the wall. It goes well for a while, but eventually they discover the Hollow Men are climbing to follow them. They kick one off and Levi begins using the sword to kill the others.
Drsa finds an axe in the toolbox and uses it to kill creatures too. While Levi pushes a monster against the wheels, Drsa has to hit one that lands on the front window to make it fall. Then a Hollow Man jumps to push Levi, who manages to grab onto the jeep at the last second but is left hanging.
He stabs the jeep’s gas tank before climbing back, then he lights a flare and throws it at the creatures. When it combines with the dripping propane, it causes a huge explosion that burns all their enemies but also makes the jeep explode. Levi breaks his leg when he hits the wall and Drsa has to help him up.
Since they’re closer to the top now, they decide to cut the rope and let the jeep fall on top of the remaining Hollow Men. Afterward the duo climbs the rest of the way and returns to Drsa’s tower. While taking care of their injuries, they agree to stay apart for five days as quarantine in case they’re infected.
They also want to run away together, so they make a plan to destroy the gorge. They share a kiss and Levi tells Drsa that he loves her before using a grappling gun that allows him to jump back to his own tower. The next day they have to do the radio check-in.
Instead of the usual employee, Levi’s call is picked up by Bartholomew because she’s learned that the computer in the lab was turned on. Since Levi denies going into the gorge, Bartholomew concludes that the person in the other tower did, so Levi’s new mission is to kill them. Levi pretends to accept the mission before the call ends.
Then it’s revealed that the lab computer had a camera and Bartholomew knows both soldiers were down there, so she calls a commander to order him to get a team ready to visit the gorge. The next morning while the soldiers are getting some mines ready for their plan, Drsa writes “I love you back” for Levi. At that moment they see a helicopter coming so they immediately start running through the forest.
When the soldiers find the towers empty, they send out drones and a chase begins. The drones quickly open fire so Levi and Drsa shoot back, bringing them all down one by one while getting a few injuries in the process. Once they reach a spot high enough, they get into their sniper positions and shoot the cloakers.
Thanks to the mines they installed earlier, they create a chain reaction that starts destroying all the cloakers in the area. Since the gorge is now exposed, the Stray Dog protocol activates. Bartholomew and her team rush to the helicopter to fly away while Levi and Drsa run as fast as they can.
The nuclear bomb goes off and the enormous explosion brings down the helicopter, making it crash and killing everyone inside. Unfortunately Levi is pushed by the explosion too and he falls off the edge of a cliff into a river. Drsa manages to escape and hides in a cave to wait for Levi.
After a month of no news from her boyfriend, Drsa decides to leave alone. Sometime later in France, Drsa is working as a waitress. One day she’s shocked to find Levi sitting at a table.
He explains it took him a while to recover from his injuries before they reunite with a kiss.
