Amy’s car stops in the middle of nowhere, and she’s unable to reach her mechanic. A truck stops where she is, with the driver offering to give her a lift to wherever she’s going. She ignores the man and suddenly sees a car right behind the truck.
She goes to the car, and the driver, Eric, agrees to give her a lift. After moving around for a while, Amy thanks him for helping her.
Eric says it’s not a problem for him, since he’s going through the same route as well. He also tells her that she shouldn’t have ignored the truck driver, because he might’ve had genuine intentions.
While trying to adjust her seat, her hand feels something in the back seat. As she brings her hand back to her side, she sees blood on it, and immediately looks scared as she wonders who Eric really is.
Before she can do anything, though, Eric suddenly stops the car, and starts beating her.
Elsewhere, a group of people are on a bus headed to the city. One of the passengers, Andrew, suddenly starts narrating the story of a serial killer who kidnaps and kills women along the highway.
The bus driver, James, believes Andrew’s story is made up, though Andrew says he doesn’t have any reason to lie. Shortly after, it is reported on the news that there has been an accident along their route.
The reporter advises commuters to go through another route, to avoid getting stuck in traffic. With this, James asks the passengers what they would like him to do. A man named Albert asks if James knows another route, and he says he does.
He, however, says it’s between mountains and the road is not good, none of the passengers are against him taking the alternate route. James heads in the direction of the bad road. As he continues driving, he keeps complaining about how he would’ve liked to pass the normal route and get stuck in traffic, instead of passing a place that would damage his bus.
Not long after, the bus goes through a tunnel along the alternate route, and before they go in too far, a man named Peter appears in front of the bus and asks them to stop.
He also seems to be heading in the same direction, so he gets on the bus, with James telling him to take his seat, and pay when he gets to his destination. As Peter takes his seat, Albert sees a picture of him in the newspaper and starts to look suspicious.
Meanwhile, a pregnant lady named Grace, who is also on the bus, sounds angry on the phone as she tells the person she’s talking to, that she’ll take care of the baby herself. She angrily throws her phone out of the bus after taking the call.
A girl named Jenny is also seated next to her autistic brother, David, who is playing with a Rubik’s cube.
After a while driving in the tunnel, Albert asks James if he has passed the tunnel before. James says it’s his first time, and Albert says it’s looking unlikely that the tunnel will eventually lead to the road.
He says it’s possible it leads to a dead end, because they’ve not seen any cars since they entered the tunnel.
Albert also blames James for passing the route, when he wasn’t familiar with it. With that, he suggests that James turn the bus around, so that they’ll be able to leave the tunnel, since they’ve been driving for 20 minutes without any exit to the tunnel in sight.
James does this, but after 30 minutes of driving back, the passengers figure out that something is wrong with the tunnel. Andrew starts to complain that he would’ve preferred to be stuck in traffic than pass a tunnel he doesn’t know.
James says it’s not his fault because he asked all the passengers, but none of them said anything, or protested against passing there, even though he implied that he didn’t want to take the route.
Just then, Grace comes out of the bus, and sees her phone that she threw out the window a while ago. This is shocking to her, because she says she threw it out not long after they made it into the tunnel.
As the other passengers try to understand what this means, David confirms that Grace threw the phone not too far from the tunnel entrance. He says that he counted the distance between the lights in the tunnel, and as they moved in, Grace threw her phone out at about 246 meters.
Albert tells the others that judging by the size of the mountain, the trip through the tunnel shouldn’t be more than three kilometers. However, James says he’s sure they’ve gone more than 30 kilometers. With this, Andrew asks if they’re being haunted.
Albert says they need to do a check-up of the tunnel, to be sure that everything is alright. He tells the others that he’ll have to get a long rope, and walk with it to see how far they can get.
As they wonder where they’ll get a long rope, Grace says she has her knitting wool in the bus, which they can use.
Peter brings it out of the bus, and after giving one end to Andrew, Peter and Albert start pulling out the wool as they walk forward.
They continue walking, and after a while, Albert asks Peter why he doesn’t look scared of what could possibly be happening to them. Peter says there are many of them in the tunnel, so he doesn’t have to be scared.
The men keep moving, and soon after, the wool reaches its end. From where they are, the men see a fishing line on the floor, and they attach it to the wool so they can go a bit further.
Meanwhile, Andrew is worried that he has still not felt a pull from the wool, and he wonders if something might’ve happened to Peter and Albert.
Shortly after, everywhere starts to get foggy, and while a girl named May is checking the mirror, she hears a sound from behind them. As they look worried and wonder what it might be, they are surprised to see that it’s Peter and Albert.
With this, the passengers believe they’re in an endless loop tunnel, because that’s the only explanation for what just happened.
To confirm that the two ends are actually connected, Peter drags it, and surprisingly, Andrew feels the pull.
The passengers start developing theories about why they’re in the loop tunnel. While May believes they might’ve taken a drug that allowed all of them to be under an illusion, Jenny and David believe there’s some science related to what happened.
They soon forget about this and start thinking of what to eat. As they gather all that they have, Andrew asks James to bring his own food and drinks.
James gets violent and tells them he’s not sharing. Albert then offers to pay for some of it, and he agrees. They all then sit at a round table to eat, and talk about how they plan to leave the tunnel. Albert tells them that they need to check the tunnel again, and he soon leaves with Andrew, Peter, and James.
After a while, the others fall asleep and wake up, only to see that the bus is now dusty. Albert and the other men also return to figure out that time has passed, and they might’ve been gone for a really long time. Suddenly, they all complain about being hungry again.
By the time James returns with what’s left of his stash, they all see that everything has spoilt, and with that, they realize that time is moving very fast in the tunnel.
While still checking around, Albert sees a map of the area inside the bus. Andrew and James argue about how it got into the car. However, Albert starts figuring out where they are and how to get out. Suddenly, the lights in the tunnel go off, and when they’re able to turn on a lighter, they see that Albert has disappeared.
They also see a blood trail, and Peter follows it until it leads them to a wall that gets broken to reveal a door. The passengers go through, and as they check around the room, they find several things.
Peter sees some engineering drawings of the tunnel, which he believes can help them. David also finds some glass jars with severed hands in them. As he sees it, James immediately points out to the others that the serial killer Andrew talked about on the bus earlier, has a similar style of operation.
James also suggests that Andrew knows something about what’s happening to them in the tunnel. Andrew denies any involvement, and says he only heard the story and shared it with them.
Shortly after, Peter says that the engineering plan shows that the tunnel has 6 doors, they’ll have to look for before they can exit the tunnel.
He says the first one is the one they just found after following Albert’s blood. They start looking for the second door, and shortly after, they find an exit back to the tunnel road.
As they come outside, James continues telling Peter how excited Andrew was, when he was talking about the serial killer. When Peter then asks James if he thinks Andrew could be the serial killer, he says Andrew is a coward.
As they continue walking, James sees his bus and goes to check inside. He finds some food and drinks, but Andrew comes in to drag it all from him before he can do anything.
Just then, a gun appears on the bus, and James uses it to threaten Andrew to give him back all that he took. Andrew is stubborn at first, but he eventually gives everything up. Soon after, Peter goes to advise James that it’s best for him to share the food with everyone, because they still have a long way to go before they reach the 6th door.
James has a change of heart and gives everyone something to eat, with Andrew even praising him and asking all of them to split into groups to find the next door.
But this appears to be a plan by Andrew to steal from James’s remaining stash. James finds this annoying, and just as he beats up Andrew, someone appears behind him and stabs him with a metal rod.
Andrew immediately runs to call the others, and it doesn’t take long for May to blame it on Andrew, as she believes he killed James just for food.
Andrew says he didn’t do anything, and they have to believe him that there is someone else with them in the tunnel who killed James. After he says that, he goes to get James’s food, and as he drags it with May, he discovers another entrance.
When they get in, they see some jars with just a finger in each one. The passengers soon discover that all the fingers have rings on them, and Peter points out that the serial killer goes after only married women.
Shortly after, the TV comes on, and they see Amy asking for help, as water enters the cell the serial killer put her in. The passengers are shocked to see this, but they don’t know what to do or whether it’s real.
As they leave the room and continue walking, they see James’s bus again. When they go closer, they see that the bus is now infested with snakes and insects.
Close to the bus, they see a table with some food, and even though it’s rotten, Andrew still tries to eat it. As he does, he gets irritated and starts complaining about how they’ll all die of hunger, because there’s no more food for them.
Just then, Grace offers Andrew some of the cookies she had been keeping for herself. After he eats one, he’s not satisfied, and he asks for everything.
Grace says she can’t give it to him, because she needs it to feed herself so she can be strong for her baby.
As he tries to drag it from her, Peter stops him, but he’s able to find a gun and use it to threaten them and take all the food items each of them has been keeping. Not long after, they see another door, and all of them go in. While checking around, they see the TV come on again, with Amy still crying for help.
Andrew shoots the TV and tells them to stop wasting their time. He says he knows why they’ve been unable to find a new door leading to their exit. He says that at both point when Albert and James died, they were able to open two new doors.
The others are confused by this, but May asks if he means that one of them has to die for a new door to be opened.
Andrew confirms this, and asks one of them to make the sacrifice for the group. None of the other passengers agree to this, but as Peter defends them, Andrew asks him about the countdown watch on his wrist, and what would happen if the countdown ends.
Peter says he doesn’t know, and whatever happens will depend on Andrew. Andrew doesn’t understand this, but Peter starts asking questions about the serial killer story he told on the bus and how it ends.
Andrew starts to feel overwhelmed by the questions, and Peter takes advantage of this to beat Andrew and take the gun from him.
While he’s on the ground, Andrew says he can’t tell Peter how the story ends, though he confirms that one of the passengers is the serial killer in the story. As Peter looks distracted, Andrew picks up a broken glass and uses it to threaten to kill Grace and her baby, if Peter doesn’t return the gun.
Andrew gets the gun back and uses Grace’s hands to pull the trigger to shoot Peter. As he then teases Peter about hearing the end of the story as he’s about to die, Jenny tries to help, but her efforts are not enough. Andrew then picks her up and takes her to another room, where he kills her after trying to drown David.
Andrew then moves on to May to kill her, but he gets stabbed with a sword that comes out of nowhere. To help David take revenge for his sister’s death, May hands over the sword to David and uses his hand to stab Andrew a couple of times.
With that, the surviving passengers, including Peter, who is still alive, keep walking.
They find James’s bus again, and while Peter goes in to take a break from all that is happening, May enters and tells him to reveal all that he knows about the tunnel.
Peter says he doesn’t know anything, but she continues to threaten him. As she’s about to kill Peter, she suddenly starts hearing the voice of Andrew laughing. As she looks out the window, she starts hallucinating and sees Andrew laughing at her.
She runs out of the bus and tries to hide, but she soon meets her death, when she finds herself in front of a mirror.
Elsewhere, Peter is able to take Grace and David toward an exit that finally appears in the tunnel. As he looks excited and turns to tell Grace that they can now escape from the tunnel, she stabs him, and he falls to the ground. Just as he watches his countdown clock run down to zero, Peter passes out.
He then happens to wake up in the real world, and it turns out Peter is a psychotherapist, who is exploring Eric’s brain to find out where he has hidden Amy, who is his wife, and Eric’s last victim before he got into an accident and suffered serious brain damage.
His adventure into Eric’s brain is for him to get the address of where the serial killer has locked his wife, so that he can save her before something bad happens.
While sitting in the cafeteria to keep working on how to get what he needs from Eric, a senior police officer comes over to tell him that they found some extra details about the serial killer.
He tells Peter that Eric’s mother and father died on the same day, with his mother killing his father before ending her life, too.
Peter then realizes that each character in the tunnel represents the different personalities in Eric’s life. After he sees a picture of Eric’s parents, he tells the officer that they look just like Andrew and May, both of whom were dangerous characters in the tunnel.
Peter also reveals that the endless tunnel loop in Eric’s brain was created by Eric, to be a reminder of how chaotic his life was while he was conscious.
When a report gets to the senior officer that a heavy rain is coming, and it might be too late to save Amy before she drowns in her cell, just like in the video from the tunnel which happens to be real.
Peter is strapped up again to head into Eric’s brain. When he enters this time, he sees Eric, who then explains everything behind his actions.
He says his mom ruined everything by killing his father and abandoning him, and since then, he has been going after women who cheat on their husbands, and also abandon their kids.
Eric also tells Peter not to worry about his wife, because she’s a bad woman. Peter doesn’t care about this and keeps moving, until he sees David.
He asks for the address to Amy’s cell, but when David replies, he does so together with Jenny, who happens to be alive again.
Just then, Peter realizes that Jenny has been the criminal personality, that has been telling David to do all that he has been doing to the women. Peter appeals to David, who is Eric’s calm personality, to give him the details of where his wife is. Just as David is about to tell Peter, Jenny drags him away.
Peter gets frustrated and suddenly finds himself in a theater, where David tries to show him that his wife is a bad woman. Peter says it’s only an illusion, but David tells him to stop lying to himself.
Suddenly, a split personality of Peter appears and claims responsibility for all the deaths in the tunnel.
He also reveals that Peter is actually the one who can’t get out of the endless loop. The evil personality of Peter tells him to forget his wife, and stop pretending not to hate her. In that moment, Peter remembers a fight he had with his wife, and it happens to be that they were not happy together.
Peter’s evil personality then convinces David to try to kill someone who looks like Amy. David is about to do it, but suddenly can’t bring himself to kill her.
Peter sees that David doesn’t really want to kill anyone, and he tells him to leave the tunnel and become conscious in the real world, so that he can tell the police where he hid Amy.
Peter’s evil twin tries to talk him out of it, but David finally leaves.
Peter then has to fight his evil twin, so that he can leave the tunnel, too. Despite beating up his evil twin, he appears to remain unharmed and tells Peter that he’s the stronger one. He also tells Peter that he’s going out of the tunnel to start a new life, to replace Peter’s real personality.
Meanwhile, Peter is not going to let this happen, and he takes a gun and attempts to kill himself. His evil twin tells him that it’s not worth it to kill himself, so he can get rid of his other personality.
He is also told that he’ll be stuck in the loop if he kills himself. Peter doesn’t care, and he shoots himself. As a result, his evil twin also fades and disappears.
Elsewhere, Eric wakes up and gives the address to the police officers just as Amy is now drowning in her cell. A while later, she gets rescued and looks happy to finally be out of there.
However, Peter remains unconscious on the hospital bed, as he remains stuck in the loop and gets to the starting point, where he stops James’s bus.