In the near future, sinkholes begin appearing all over the world causing explosions that send society into an utter state of panic. Huge monsters known as “reapers” come out of these holes and chase after humans to kill them. Bullets don’t hurt them, so not even the military can save society.
Eventually people who live in high altitudes report that the monsters freeze at eight thousand feet, so survivors start moving to elevated areas to start a new life. Three years later in Colorado, Hunter lives in a small community in the Rocky Mountains. One day he decides to cross the eight-thousand feet safe line, curious to see what’s on the other side.
He barely gets to take a few steps before he encounters a reaper, so Hunter begins running back. The monster almost gets him but he manages to cross the line just in time. Afterward Hunter returns to his home in the Lost Gulch Refuge where 193 survivors live.
His father Will scolds him for crossing the safe line, reminding him of the monsters but also of Hunter’s lung problems. Hunter explains he only wanted to find more people, so Will points out he can’t lose him like he lost his wife. Later Will goes hunting with Katie and shares what happened.
Katie understands his worries but also points out Hunter is the only child in their community, so it’s natural that he wants to find friends to play with. However Will explains Hunter is just trying to go to the last place he saw his mother. Meanwhile scientist Nina is experimenting with bullets and tests them by shooting them at a piece of reaper hide, but they still don’t do any damage.
That night Will hears Nina still shooting and gets annoyed as he remembers the time his wife left with her on the special mission that killed her. Suddenly he hears Hunter coughing and rushes to the boy’s room to change the filter of the machine that helps him breathe. Soon Hunter calms down and can sleep again, but that was the last filter in the house.
Will stares at some maps, wondering how hard it would be to cross the line to go looking for more filters. On Hunter’s birthday, Will manages to get the community’s last box of mac-and-cheese to give his son a treat. While they cook, Hunter wonders why they don’t use the radio anymore.
Will explains they need to save electricity, so they use flags to communicate with other refuges. Hunter thinks this is sad because talking to people is important. That night Hunter begins coughing again and Will has no choice but to give him an injection.
After Hunter falls asleep again, Will goes to see Nina to tell her he’ll leave for Boulder in the morning to look for filters for his son. He claims he’s found a safe road, but Nina doesn’t believe him because she has the area memorized. It turns out her old lab is in Boulder.
Will wants her to come with him because she’s studied the reapers for years and survived the last trip she made, however Nina points out she only got lucky and Will’s wife did die during that trip. Refusing to give up, Will tells her about a mine that goes through the mountains, yet Nina thinks that’s worse because the reapers can trap them there and still refuses to go. After Will leaves, Nina works on modifying a compass but she can’t stop thinking about the death of Will’s wife, which leaves her in tears.
Meanwhile Will asks a neighbor to take care of Hunter before going to the gun store to get a bunch of weapons. Katie shows up and announces she’s going with him, not caring about his protests. Nina also gives in and agrees to come while mocking Katie because she has no experience and will be a burden.
Will says goodbye to Hunter and promises he’ll come back, but the boy points out his mom said the same thing. The trio finally leaves and Nina continues to be rude to Katie, so Will tells her to behave. Nina then explains that reapers emit high bioelectromagnetic pulses and her modified compass can detect them when they’re within a half mile.
After a long walk, they find the Elba Fire Road and the eight thousand feet line. Once the compass confirms there are no monsters nearby, the trio crosses the line and starts going down. Moments later they find some abandoned trucks so they search them for supplies.
They get some flares, ammo, and even a grenade launcher that Katie knows how to use. Nina also finds a reaper scale and takes some time to gather a sample. When Katie tries to rush her, an argument ensues and Nina accuses her of trying to get into Will’s pants.
A furious Katie pushes her against a truck and reminds her that Nina killed Katie’s best friend, referring to Will’s wife. The women start punching each other and Will has to separate them before it gets worse. The group keeps traveling and is surprised to find a bunch of horses in the forest, which means the reapers only attack humans for some reason.
Eventually they make it to Alta Vista Ski Area, where they plan to use the chairlift to travel to the mines safely. At that moment Nina’s compass indicates that there’s a monster nearby and they see something shaking the forest trees. Will immediately climbs to the top of the lift and starts working on activating the backup generators while the women stay on the ground to offer cover.
Katie panics when she sees the monster approaching and shoots a few grenades that cause some explosions but do no damage at all to the reaper. Will finishes connecting the generator and Nina hits the switch before getting on a chair, where Will immediately joins her. They open fire at the reaper as it gets closer while Katie begins climbing the tower to reach the chairs too.
The monster jumps on the cables and makes the chairs shake, but the duo manages to hold on. Katie waits for the right moment to jump but unfortunately misses the chair, so Will has to catch her mid-air. Nina runs out of bullets right before the monster makes the cables shake again, causing all three of them to fall to the ground.
They immediately start running and manage to cross the line into the eight-thousand feet area right before the monster can catch them. Since it’s getting dark, the trio decides to stay in the ski resort for the night. While they share a drink, Nina points out that when the reapers appeared, they killed 95% of the world’s population without stopping to eat or sleep.
Predators kill to eat, but the reapers kill humans for fun and don’t attack other animals. This makes her wonder if they were sent specifically to get rid of humanity. After the women fall asleep, Will remembers again the last talk he had with his wife, who really trusted Nina’s skills to find a way to kill the reapers.
The next day the group finally makes it to the mine where Will used to work. He explains that both ends of the tunnel are above eight thousand feet but the lower levels do go below the safety line, which isn’t received well. The tunnels aren’t properly sealed so the creatures could still find them, not to mention Nina’s compass won’t work underground.
They’ve come this far though, so Nina and Katie have no choice but to keep following Will. Wearing construction helmets for light, the trio enters the tunnels and spends a few hours walking in silence. Unfortunately they’re forced to stop when they discover the main tunnel has been closed because of a cave-in.
Will announces they’ll have to go down level seven, which means they’ll walk under the line for a thousand yards. After getting startled by some bats, the trio takes the stairs and enters the new tunnel, walking as fast as possible to avoid trouble. Suddenly they hear a noise and cover the lights on their helmets, but it’s too late: there’s a reaper in the tunnel and it’s already seen them.
The group immediately starts running while Katie shoots the occasional grenade, hoping to slow the monster down. Abandoned machinery also keeps the reaper back and Will opens fire, but it’s pointless. When they find a hole in the wall, the trio runs inside and Will shoots at the rocks to make enough room for the three of them to hide.
The reaper inserts its glowing tail inside the hole so Will throws a flare to distract it. The light reveals a way out in the opposite wall, so they push away a few rocks to crawl through it to escape. Will moves last and has to dodge the reaper’s tail before following his friends.
After crawling for a few minutes Katie comes out into another tunnel, only to be found by a reaper that immediately stabs her with its tail. Will tries reaching for her, but he isn’t fast enough and has to watch how the monster takes her away. At that moment Nina finds a ladder, so she and Will immediately climb up while hearing a reaper behind them.
Thankfully they find the door and run out of the mine, reaching a safe altitude just in time. Nina and Will continue their journey, not stopping for anything except petting a goat. When it gets dark, they find an abandoned house and break in to spend the night.
There are some mac-and-cheese boxes and they have them for dinner while Will shares that it’s Hunter’s favorite food. He asks Nina what’s in the old lab, so she explains she may find a chemical that may finally help the bullets damage the monsters. She theorizes that the reapers’ scales are electrically charged, so the right bullet could cause internal combustion.
Will thinks it sounds like an unbelievable magic bullet and can’t believe his wife died over a weak theory. The next day they take the truck from the house and use it to cross the line as fast as possible, going through a town that has been taken over by animals. Eventually they make it to the hospital and search for the filters while hearing some weird noises, but the compass doesn’t alert them of anything.
They have to go to the last floor to finally find the filters, then they start running to escape. Suddenly a reaper pierces a wall and the duo has to move quickly to dodge those huge claws. They still bump against a bunch of furniture on their way out and get hit by falling debris every time the reaper destroys a wall or window.
Suddenly the monster’s tail reaches through a wall and captures Will, so Nina grabs the fire ax and hits the tail with it until the monster lets go of her friend. Then Nina opens a gas canister to fill the corridors with fog, hoping it’ll provide some cover. This gives Will an idea so he waits for the reaper to come close enough and shoots a bunch of oxygen tanks.
It causes an explosion that sends the monster flying through the window. The creature isn’t moving anymore but they can’t be sure if it’s dead or unconscious, so they need to hurry. Will wants to go back, however Nina convinces him to help her get to the lab so she can finish her bullets.
After the duo leaves on the truck, the reaper wakes up. Moments later they make it to the lab and Nina starts mixing chemicals while Will keeps watch. On one of the desks he’s shocked to see a picture that reveals Nina used to have a family she never talks about.
Nina cries as she explains that on the day the world ended, she had been working and couldn’t be with her family. She feels extra guilty because her son had a soccer game that day and she chose not to go. Nina also admits she hates herself for causing the death of Will’s wife, but Will assures her it was nobody’s fault.
When Nina finally finishes her bullet, she tests it on the hide and nothing happens. She tells Will to go back to his son but she’s staying to keep on experimenting until she finds a solution or the reapers kill her. Will leaves on his truck while Nina tries another bullet mix, but it doesn’t work either.
She’s so distracted that she doesn’t notice her compass reacting. At that moment she hears some noise but also realizes there’s a certain chemical combination she hasn’t tried, so she hurries to mix it as her last hope. Meanwhile the truck accidentally pops a tire and loses control until it falls on its side.
Will is unconscious for a few moments but wakes up when he hears a monster coming. He has to kick off the truck window to get out and starts running as the reaper chases after him. Going up the hill is hard for his body after the crash and by the time he’s about to cross the safe line, a second reaper is blocking the way.
Will desperately opens fire to no avail and loses hope, but at that moment Nina shows up and shoots her new bullets. A strange explosion happens full of smoke and electric charge, confirming her theory. Nina quickly shoots all the reapers, killing them in seconds even when they try to run away.
Then she grabs a piece of the reaper's body and shows Will the mechanical parts, confirming these beasts are robots that were programmed to kill humans. When Will asks who sent them, Nina theorizes it was aliens. Later when the duo returns to their refuge, Will reunites with Hunter and Nina raises a pirate flag to announce she’s discovered a way to fight.
Soon survivors gather to build tons of special bullets and Will finally uses the radio again to send their message of hope everywhere. Nina also keeps monster bodies to keep on studying them. Once everyone is armed, the survivors enter the forest and kill as many reapers as possible to retake the land.
One night Will and Nina look at the sky with a telescope and notice three mysterious lights approaching Earth.
