In the forest, a search party is looking for a missing couple. In an underground cave with a bell on the ceiling, two search dogs find water in a creepy hole and decide to drink. Moments later, the owner notices those two dogs staring at each other. That night, the dogs are put in their cage while they continue to stare.
Later, the owner wakes up when he hears some strange noises and rushes to the cage to investigate. To his shock, he discovers that the dogs have fused in a very horrifying way. In the city, teacher Millie and failed musician Tim are having a party since they're moving away soon. Tim's former bandmates tell him that they've been hired for a tour, and he agrees to rejoin the band, even if it means he has to drive a lot between the city and their new place.
Meanwhile, Milliey's best friend asks her if she's sure about her plan since Tim has been very distant and depressed since his career hit rock bottom. But Millie thinks the move will help their relationship. When the couple makes a speech to say goodbye to their friends, Millie surprises Tim by getting on her knees and asking him to marry him while using an imaginary ring.
Tim is too shocked and doesn't know what to say, making the moment too awkward. When Millie stands up, Tim finally says yes. Later in bed, Tim apologizes and explains he froze because of the shock, but Millie is still mad. She points out he's been distant and wonders if he wants to break up because it's the first time he's made her feel unsure about their relationship.
Tim assures her that he loves her and wants to stay together. But when she tries to get dirty with him, he turns her down. As they go to sleep, Millie says that everyone knows he needs her more than she needs him and even laughs about it. She also points out that resenting her won't make him less of a failure.
Tim turns around and watches her back as she asks him to get rid of them. When he looks ahead, he's shocked to see a crying woman with a dead man at the end of the bed, which is starting to shrink. Then Tim tries hiding under the covers, only to find the woman's face creepily coming for him. At that moment, Tim wakes up from this nightmare.
At the new house, Tim asks Millie if she thinks he resents her, but she doesn't know what he's talking about. The couple starts unpacking, and Tim finds his dad's power tools in a box. Suddenly, he smells something really awful and follows it to discover it comes from a light upstairs. Tim removes the lamp and inserts his hand only to find a dead rat.
He starts pulling and is disgusted to discover multiple dead rats whose tails got tangled. Then he's startled by one of the rats squeaking and he drops them all causing the last rat to die too. All the bodies are thrown into the forest. The next day, Millie starts working at her new school and befriends a teacher named Jaime who lives near Milliey's home.
He tells her that if she ever wants to explore the area, he can be her guide. In the evening, Tim asks Millie if she can drive him to a gig, but she can only drop him at the train station. When she asks him to get dirty tonight, he turns her down. The following morning, the couple goes on a hike together and gets confused when the road ends abruptly.
Tim finds a different road and sees all the bells hanging above it, so he thinks they indicate it's safe. The couple stays in the forest for a while, but eventually it starts raining, so they try to run home only to get lost. Tim takes out his phone to check the compass, but he realizes he doesn't know in what direction their house is.
When Tim moves ahead, he accidentally falls into a hole, but manages to hold on at the last second. Millie rushes to help. However, she can't pull his weight and they both end up falling to the bottom of the cave. Tim's phone breaks and Milliey's doesn't have signal. They can't help feeling like the cave is crowded and when Tim takes out his lighter, he finds some old furniture.
He quickly breaks it and tries to start a fire with a wood to no avail. There isn't much water left in their bottle, so Tim lets Millie take it while he drinks from the small pool in the ground. Moments later, they managed to start a fire and open a bottle of alcohol that Tim brought. They both admit they've started smoking again and share a cigarette to deal with the stress.
When Millie asks him how he was able to smell the rats, he explains that during his childhood, his father kept smelling something awful in Tim's room. However, no matter how much he cleaned, he never found the source. One day, Tim came home after school and found his father with a dead rat that he' found nesting above the lamp.
Every time Tim turned on the light, he was cooking the body. But it was so gradual that he didn't realize the smell was different. That same smell was in the house the day Tim found his father's body next to his creepily smiling mother on the bed. Her mind had snapped when she woke up next to her dead husband.
And just like Tim with a rat, she became immune to the smell. Tim is crying by the time he finishes the story. In the middle of the night, Tim wakes up when he hears strange sounds in the darkness. He starts having a panic attack as he listens to a beast approaching. So, he takes deep breaths and realizes the sounds come from the cave walls, which are moving in a way that mimics his chest.
As Tim feels the cave closing around him, Millie starts panting and some hands suddenly grab his face. At that moment, Tim wakes up from the nightmare. Milliey's phone has taken water damage overnight. The couple then tries to stand up, but they feel something weird in their legs. They move the blanket and are surprised to discover that their legs are stuck together by some weird sticky substance.
Pulling their legs is enough to separate them. However, the process is extremely painful. Thanks to the morning light, the couple manages to climb out of the cave and discovers that the leg pull left wounds on their skin. At home, Millie asks Tim for a massage and he hesitates before touching her back. He feels a strange connection as he starts massaging her and his fingers dig so deeply into her skin that he starts hurting her.
He doesn't snap out of it until she pushes him off. Millie notices that Tim looks sick, but he says he's just tired. Afterward, Millie leaves in the car to go downtown. Tim gets in the shower only to suddenly lose control of his body. With every move of Milliey's in the car, Tim's body turns in the same direction, causing him to hit the walls and the shower door over and over.
When Millie drives forward for a few miles, Tim's body keeps pressing against the wall until the tiles crack. At that moment, he snaps out of it and falls back with a wound on his forehead. Later, Tim goes to the doctor, who immediately comments on his mother's history with mental health. Tim insists he isn't like her, and the doctor gives him a muscle relaxant.
This medicine makes the body relax even if the brain panics, which should shut down the feedback loop. The doctor also tells Tim that he and Millie were lucky because the last couple that lived in that house disappeared. When Tim gets home, he searches the internet for information about the missing couple. It seems they went hiking and never came back.
The search party is no longer active. Tim can't stop looking at the couple's social media and asks Millie to delete his accounts when he dies. At that moment, Jaime comes for a visit and ends up staying for dinner. Tim asks him about the bells in the forest and Jaime tells him about a new age church in the area that collapsed years ago.
Millie and Tim then tease each other, but soon it escalates into actual insults. So, after an awkward pause, Jaime says they complete each other. After their guest leaves, Tim tells Millie that Jaime clearly likes her. Suddenly, Tim starts kissing Millie very passionately and he loses himself in the kiss.
As hands start exploring, Millie tries to move only to notice that their lips are stuck together. She manages to pull her face off, but it's very painful and she thinks he bit her. Tim denies it and explains something has been off since they came to this house. However, Millie thinks they're just excuses to avoid getting dirty.
Later in bed, Millie wakes up because of a weird noise. She can feel her hair stuck and thinks Tim is sleeping on top of it. But when she turns on the light, she's horrified to discover her hair in Tim's mouth crawling into his throat to choke him. She painfully starts pulling and wakes up Tim, who uses his hands to help. After lots of effort, they manage to rip the hair off and it falls on the mattress.
The next morning, Tim finds more hair in his mouth. Today, he has a gig and Millie thinks he shouldn't go since he's not mentally well. Tim insists he'd been unconscious and refuses to talk about it, so Millie drives him to the train station. There, she wishes him good luck and tries to be supportive of his career. She even tells him to stay the night with his friends so he can have a break from the new house.
While waiting for the train, Tim starts having a panic attack, which gets worse the farther Millie drives. He takes his medicine, but it doesn't help, and he can't move his body when the train arrives. Leaving his instruments behind, Tim begins walking back to town while feeling his skin burning. At school, Millie sees the drawing a student made of her dogs, which shows them fused at the face.
After class, she notices Tim appearing in the backyard looking very sick. She runs to hide him in the nearest bathroom, where he tells her how much he's missed her before kissing her. The couple gets in a stall and gets naughty for the first time in a long while. However, when they're done, they're shocked to discover that their groins are stuck together and pulling hurts a lot.
Suddenly, a kid appears outside the stall and the couple has to stay silent until the boy is gone. Then, Millie covers Tim's mouth and apologizes before pushing him off, which finally separates them, but also leaves them in a lot of pain. At that moment, Jaime knocks on the stall. So, Tim hides on the top of the toilet and Millie presses the button before coming out.
It turns out the boy brought Jaime after hearing the weird noises. Jaime reminds her to use the staff bathroom and notices the blood on her leg, so he assumes she got her monthlys. On his way out, he sees Tim's leg in the stall. When the couple gets home, Millie yells at Tim for what he did, and he points out that he didn't force her.
Millie keeps on ranting, saying that Tim is trying to ruin things for her. When Tim reminds her that he left his life behind to move because of her work, Millie says he didn't have a life in the city. If he feels so trapped, she thinks he should leave. However, Tim explains he can't because he feels like his brain and his body aren't working.
Millie leaves after telling him that she feels like his mother because she's living with something dead. Minutes later, Millie goes to Jaimes house to apologize for the incident and she can't help but cry. So, he invites her in. They talk about the state of her relationship and Jaime shares something he teaches in class.
Legend says that humans were created with four arms, four legs, and a head with two faces, but Zeus thought they were too powerful and split them into two separate parts. This doomed humans to spend their lives searching for their other half. Jaime also shares that he used to have a husband and he left him, which he deeply regrets. At that moment, Millie sees Tim outside the window and decides to leave.
Tim follows her and holds her hand while he apologizes. That night, Tim stares at all the worried messages from his band, but he isn't brave enough to answer them. He then checks the last picture posted by the missing couple during their hike and gets the location. When he puts it on the map, he realizes that they disappeared in the mysterious cave.
Tim tries to take a closer look at the picture, but at that moment, the internet goes out. Suddenly, something crashes against the door, startling Tim into falling off his seat. He's horrified to notice that Milliey's body is rubbing on the glass, and her face appears to be bleeding. Her body keeps pressing against the door as if desperate to get in.
She even starts floating up, and Tim is about to do the same. However, he manages to concentrate and open the door instead. An unconscious Millie falls on top of him, and Tim has to slap her to wake her up. Millie doesn't remember anything, so the couple moves to the kitchen to discuss what happened.
She wonders if it was a seizure or if she sleepwalked, but Tim insists something weird is going on. He thinks the cave had an effect on him and it's now affecting her too. And he wonders if they'll disappear like the previous house owners did. Tim wants to test this by putting their hands together, but Millie walks away. However, she stops outside the door and Tim says she did that because his body didn't want her to leave.
He describes a pull and a thirst in the bones, saying something inside them has imprinted on each other. Tim wants to return to the cave for answers, but Millie just reminds him that her parents are visiting them on the weekend and leaves. Then the blood she spilled on the table leaks through the wood. Afterward, the couple goes to sleep in different beds.
In the middle of the night, Tim wakes up when his body is being dragged on the floor by an invisible force. He tries to fight back to no avail, so he has to hold on to the wall to make it stop. Another door starts shaking and suddenly opens for Millie, whose body bends backwards before she falls. Her body continues to twist in various terrifying ways as it's dragged toward Tim who is being moved too.
Their hands finally touch and the couple screams in pain as their skin opens so their arms can start to fuse. The sound of cracking bones echoing in the hallway. Tim remembers his medicine and tells Millie to reach for the drawer while their arms continue to push into each other. The drawer falls near their heads, hurting Millie.
They use their free hands to open the medicine bottle and immediately take a bunch of them. Tim wonders if it'll work more quickly if they snort it, so they crush the medicine and absorb it through their noses. Next, they put their feet together to push as hard as they can, which causes them even more pain, but also starts pulling their arms apart.
Unfortunately, before they're done, they fall unconscious. The next morning, Tim wakes up to find Millie tying him to a chair with tape. Their arms are still fused, but it seems the medicine has paused the process. Both of them drink lots of whiskey before Millie grabs the buzz saw, losing control as soon as she turns it on.
Thankfully, she quickly recovers, and after some hesitation, she starts cutting. The pain is unbearable and they both scream through the whole process, but at least they managed to split. Afterward, the couple has a snack while trying to calm down. When they notice they have almost no medicine left, they decide to go to the hospital.
However, Millie realizes that she left the car keys at Jaimes and has to go back to find them. At Jaimes house, Millie doesn't notice that the bell at his door has the same symbol as those in the forest. He doesn't answer the door, so she bursts inside to look for the keys, but gets distracted by the noise coming from an old TV.
She takes a closer look and discovers that Jaime got married at the church that sank in the area. Behind the priest, there's a horrifying painting of two humans fusing together. It seems this new age church was some kind of cult because the video shows the priest and the guests taking Jaime and his husband to the hole in the cave, locking them up on purpose.
Suddenly, Millie is startled by Jaime appearing behind her wearing his cult clothes. It turns out he's a fusion of the couple from the video, but not all the couples managed to reach this perfection. Jaime thinks Millie and Tim should do the same because this is the ultimate intimacy. He touches Milliey's hand and slashes her skin with his ring, so Millie punches him before leaving the room.
The punch causes Jaimes face to start splitting into the two original people while he keeps preaching the benefits of the fusion, but Millie ignores him as she escapes. Meanwhile, Tim grabs a flashlight and some rope before going back to the cave. As he looks around, he keeps stepping on a sticky substance. On the cave walls, he's shocked to discover the ugly painting from the church.
Suddenly, a hand grabs his leg and makes him fall. As his jacket gets stuck on the sticky ground, Tim sees a creepy creature coming out of the wall. This monster appears to be two people badly fused into each other, meaning this is a couple that couldn't do the ritual well. As the creature comes closer, Tim breaks his jacket and retrieves his flashlight to flash it at the monster's face, which keeps it at bay.
The creature appears to be asking for help. However, Tim only has eyes for the knife stuck in its neck. He takes it out and cuts the monster's fingers, creating an opening that allows him to run away. Millie and Tim return to their house at the same time and try to keep a distance between them. However, the invisible force immediately starts pushing them toward each other, twisting their bodies if necessary.
The couple holds on to random objects to stop the pushing, but Tim suddenly says that he knows how to stop it and lets go. He starts walking toward Millie, and after she lets go, too, he gets on his knees to propose with an invisible ring. After they share their love for each other, Tim takes out the knife and says he'll selfdelete to stop this.
Millie tells him to stop as the blood from her arm falls on the ground and moves in a line to climb up his body. She collapses in his arms and says a final goodbye while the blood reaches Tim's face. He clings to her body, crying over her death. Minutes later, Millie is surprised to wake up inside the house. She looks down and discovers that Tim has fused their arms to keep her alive.
Tim puts her favorite music on and the couple takes their clothes off before they start dancing. When they share a kiss, their bodies continue the fusion process in the most grotesque way. It's slow and painful, but it works. The internet then comes back and shows that Tim's last uploaded photo on social media is of them at the new house.
On the weekend, Milliey's parents arrive for a visit and ring the new bell outside. The door is opened by a perfect fusion of Millie and Tim.