The film starts with 15-year-old Rebecca 'Becca' (Olivia DeJonge) interviewing her mother, Paula (Kathryn Hahn) for a documentary she's making about meeting her grandparents for the first time. Paula explains that as a teenager, she fell in love with her substitute teacher, and her parents didn't approve. Something happened when she was 19 that caused her to not want to see her parents again, for the last 15 years. She points out that her husband eventually fell in love with another woman he met at a Starbucks, and moved to Palo Alto. Becca asks her to go back and explain what exactly happened to cause the years of non-communication, and her mom tells her she won't tell her; if her grandparents want to give her that information, it's up to them. She tells them even though she hasn't talked to them in years, she knows they are nice, and they still volunteer at the local hospital.
We meet Tyler (Ed Oxenbould), Becca's younger brother, while they drive to Grand Central Station. He is age 13 and talks like a wannabe rapper, complaining that he's got three girls on deck and is upset he won't be able to text all week, due to having no cellphone reception where the grandparents live. Their mom hugs goodbye at Grand Central, and they board a train. On board, Tyler shows off his freestyling skills by rapping for the camera. Becca mentions that she agreed to the trip because their mom hasn't been able to connect with her new boyfriend, and a five-day cruise might help them get closer, (as in getting laid and having steamy, passionate, wild sex day in and day out!!).
They get to Pennsylvania where their grandparents live, and they're waiting for them as they get off the train. The grandparents, John "Pop-Pop" (Peter McRobbie) and Doris "Nana" (Deanna Dunagan), seem friendly enough and take them back to their house. Tyler does a freestyle rap using Nana's suggestion of pineapple upside down cake. Becca discusses her documentary and her love of making movies.
Tyler and Becca get settled into their room upstairs, which used to be their mother's. They play rock, paper, scissors to see who gets the bed and who gets the sofa, and Becca gets the bed. She tells Tyler about the old time song she's going to play over some of the footage when there's a happy conclusion to the week. She gives Tyler a second camera so he can film additional footage.
Tyler films Pop-Pop mysteriously working in the shed. He calls out to him and Pop-Pop sees him but doesn't respond.
Tyler coerces Becca to play Hide and Seek underneath the house. They crawl around, and then suddenly Nana is down on all fours behind Tyler. She races after him, and then Becca, each scurrying to get away from her as she seems demented and off, repeating "I'm going to get you" as she scurries after the kids. They escape from underneath the house, and Nana laughs, her hands sullied, seemingly aware of the game and simply trying to participate. She walks away, revealing the roughhousing has caused her dress to ride upwards, exposing her bare butt.
A man comes to the door and asks to talk to their grandparents. They tell him they're not there. He says he knows them from Meadow Shade, the hospital they volunteer at a few days a week, and he has some gossip to tell them about the latest drama going on down there.
Tyler decides he's going to investigate what's in the shed. He sneaks inside and says it "smells like ass". He finds in the corner a pile. He gets closer to see what it is and discovers it's used adult diapers. He runs out screaming. Inside, Nana explains to him that Pop-Pop is incontinent, and a lot of adults have to wear diapers. He hides them in the shed because he's ashamed, then he burns them. She then continues giving Becca tips on how to make cookies.
That night, Pop-Pop comes into their room and tells them that there is mold in the basement, and they should not go down there. He also tells them that everyone follows the same schedule, so lights have to be out at 9:30. They agree but are annoyed, especially since there is no WiFi, and they can't use any electronics. Tyler decides to start using pop stars names instead of swearing in his raps and says if he stubs his toe, it sounds cooler to shout out "Shakira!" than a cuss word (This is a motif that is carried out throughout the movie with him shouting out Sarah McLachlan and Katy Perry in times of annoyance or danger). The two can't sleep, and it's now 10:23 PM. Becca says she's going to sneak out to get one of Nana's cookies. She opens the door and sees Nana walking in the dark, projectile vomiting. She quickly shuts the door.
The next morning, Pop-Pop and Nana are outside with breakfast on the table. Nana apologizes because shes got hot oil all over Becca's computer but really only the webcam. Becca says she will probably be able to scrub it off with enough effort. The kids later ask Pop-Pop if Nana is sick. They are told Nana experiences something called "sundowning", which is a form of dementia that happens when the sun sets. It's the equivalent of talking in one's sleep and not to be concerned, but it's best for them to stay in their room during the night. He says Nana is convinced there are bad things inside her, so she throws up to get rid of them. As he's explaining this, he's putting on a tuxedo. They ask him if he's going somewhere, and he tells them there's a costume ball at the train station he's late for. He then realizes that he's confused and takes the tuxedo off.
Pop-Pop takes Becca and Tyler through the town. They play a game where they make up stories about people who live in the buildings including the closed police station. When they try to make up a story about a tall building, Pop-Pop tells them its Meadow Shade where they volunteer, and he'll show it to them when he gets his Meadow Shade badge from home. They go to the park to play, but Pop-Pop tells them they have to leave because they're being followed. The kids see a man across the street using his cell phone, not paying attention to the three of them. Pop-Pop runs over and begins to assault the man, yelling at him. Becca and Tyler convince him to leave the man alone, and Pop-Pop apologizes to them.
Back home, Becca is in the kitchen with Nana. She asks her if she can interview her, but Nana does not want to be on camera. Instead, she asks for Becca's help cleaning the oven. Becca cleans with just her arm, but Nana tells her to lean into it. Nana then convinces her to get completely inside. While she's fully submerged in the oven, Nana bounces up and down excitedly. Becca reappears, and Nana tells her she is ready to be in her movie.
Becca interviews Nana by asking her warm-up questions. When she asks Nana what happened 15 years ago to cause her not to speak to her daughter, Nana starts going berserk, shaking violently, and screams that she no longer wants to be in Becca's movie.
Outside, Tyler interviews Becca asking what animal shed want to be (a dolphin), then why she likes the pizza guy despite him having bad acne (he has kind eyes.) Then he asks her why she can't look at herself in the mirror, pointing out when she brushes her hair, she does it with her back to the mirror. And when she brushes her teeth, she looks down. She hints that it's because their dad abandoned the three of them years ago, and she has felt rejected. Tyler defends his dad, saying there was a time when he was eight when he was playing peewee football. His team was up by three, and it was the fourth quarter and they were set to win as long as nobody scored in the final minutes. A big kid came running towards him but instead of blocking him, he just froze. Everyone started screaming at him but he was completely frozen, immobile, which is what happens when he's afraid. But his dad never judged him for it. But he sometimes blames that for being why his dad went away.
In the editing software, she's piecing together on her computer, Becca films herself in front of an obstructed slideshow of pictures of her brother, her, and their father. She says that while she's trying to tell the story of her mom's parents, she will not be including anybody from the past that she doesn't consider worthy of acknowledgment.
That night, at 10:47 PM, they hear a scary sound coming outside their locked door. The two want to film what's on the other side, so Tyler tells Becca to open the door. She refuses. He then says if she holds the camera, hell open the door. He does, and they reveal a naked Nana clawing at the door opposite them, scratching like a frantic dog. He shuts the door and declares that he's now partially blind.
The next morning, Becca interviews Pop-Pop, and he tells her how he used to have a great job, but he used to see a white figure with yellow eyes at his job. Nobody else could see it, but he was insistent it was there. So he was eventually fired. He warns Becca that she, too, will see the white figure with yellow eyes one day. She tells him he seems sad.
Tyler tries to convince Becca to set the camera up in the living room so it can film what happens at night. She says she can't film their grandparents unless one of them is there otherwise it's unethical. She explains they're both experiencing signs of early onset schizophrenia.
A neighbor named Stacey comes over, telling them their grandparents volunteered at the hospital when she was in rehab, and she baked treats to thank them.
The kids get an Ethernet cord and now talk to their mom on Skype. Tyler tells her Nana is acting weird. The mom tells them "they're old, and that's just how old people act". Becca defends them and says they are weird but nice. Tyler and Becca both agree that this is a 1 on the scale of problems. Their mom comments how she wishes she could see them (but can't because their webcam is blocked from Nana's mishap in the kitchen). Their mom leaves to watch her boyfriend in a Hairy Chest contest on the cruise ship.
That night, at 10:16 PM, they hear an odd commotion outside the door. They want to know what Nana is doing this time but are too scared to look. Becca decides just to open the door and film for a short while, for the documentary sake. When she opens the door, they see Nana running past, with both arms behind her back, rushing past them, in both directions. Just as she's about to crawl towards the camera, they shut and lock the door.
The next morning, the four of them go out into the woods. Becca says she doesn't want to leave without getting an elixir for Mom. While the grandparents are ahead on the trail, Tyler begins to mimic Nana's running with her arms behind the back only to get caught by Nana, who tells them they're going to miss the family of foxes. They turn the corner and see Nana staring into a well. They ask her what she's looking at, but Pop-Pop tells them it's nothing.
Tyler and Becca return to the well later to try to figure out what is hidden inside. But all they pull up is water.
Becca goes in the shed and finds Pop-Pop with a rifle in his mouth. He declares he's just cleaning it and then mimics cleaning it.
Later that evening, Becca is in the living room and hears Nana laughing hysterically. She decides to show what kind of television show makes her Nana laugh, hoping it's the same one her mom loves. But she finds Nana rocking in a chair, facing the wall. She asks Nana what she's laughing about and is told the naughty spirits are inside her, and she laughs to keep them at bay. She then tells her a story about how there are people in the water that were stolen by people from another planet. These people will later be collected and sent back to this planet but for now, they're at the bottom of water. Becca tries to interview her again, but she goes crazy when she is asked about the night that caused them to become estranged. When Becca presents it as a story about a girl who fell in love with an older man, whose family did not approve, and what she would say to the girl, Nana tells her I would tell the girl "I'm sorry."
Becca now has her elixir, an apology from Nana. Outside the window, they see Nana and Pop-Pop in a heated argument with their neighbor, Stacey. They wonder what they are fighting about.
Becca decides Tyler is right and that they should set up the camera in the living room to see exactly what goes on at night. Becca also wonders what's in the basement given that they were told not to go down there.
At night, Tyler is freaking out because he touched something slimy on the toilet handle and can feel it seeping into his skin. Becca gets tissues and helps wipe it off. Time passes, and they fall asleep. In the living area, Nana opens and slams the basement door several times. She then rushes around the room, crawling like a dog then appears..... RIGHT IN FRONT OF THE HIDDEN CAMERA and screams. She picks up the camera and then films herself going into the kitchen where she grabs a butcher knife. She makes her way up to the kids' bedroom and begins pounding at their door. Becca and Tyler wake up, startled. They can hear Nana trying to get in but just stay still.
The next morning, they watch the footage and see that Nana was trying to kill them. Becca tells them that their mom is back from her vacation that day so they just have to avoid their grandparents all day until she can come and get them. They throw the ball around and every time the grandparents come by, they tell them "We're playing. This is how kids play."
Inside, they try to avoid their grandparents by going out to play but Nana asks if Becca can help clean the oven first. Becca leans in, but Nana tells her to go in further. Tyler objects but Nana tells him they've done this before. Becca finally climbs all the way in and Nana pushes her fully inside and shuts the door, telling her she wants to do something real quick and wipes down the handle. Tyler screams at her to open up the oven, and she does. Becca is shaken up, and they quickly go outside and play.
They wait until the grandparents are out front and then get on Skype, hoping to sneak in a call without the grandparents being aware. The oil has now been scrubbed off of the webcam so their mom can see them, too. The mom is back home and tries telling them about her vacation and a fight with her boyfriend, but they quickly tell her that she needs to come and pick them up right now. She tells them, "Do you know how long it'd take to drive from here to there?" but they tell her to get in the car immediately and make her way to them. They say that their grandparents are scaring them; Nana tried to kill them with a butcher knife, and Pop-Pop put a gun in his mouth and she's afraid he's going to hurt himself.
Tyler films the grandparents from the window so his mom can see them. The mom is now white-faced and tells them she has to tell them something and for them to listen. She says: "THOSE ARE NOT YOUR GRANDPARENTS!!!!" She asks if they've been staying with them all week and tries to call the local police but gets a recorded message (the station is closed). The mom complains that the hick town has an incompetent police department, and she's going to drive to come get them and will continue to try to call the police on the way. Heading out, she tells them to get somewhere safe but just then the grandparents return, and they shut down Skype. The grandparents suggest having a board game night, but the kids say they want to check something outside while the grandparents figure out the teams. They head for the yard only to see..... STACEY HANGING DEAD FROM A TREE! Nana appears and tells them they already have the teams... Old versus young.
The kids are forced to play Yahtzee with the fake grandparents, who eerily pretend everything is normal, Nana complaining how competitive Pop-Pop is. They begin to play the game, but the grandparents are becoming more demented. Pop-Pop begins dressing up for the costume party again. Becca excuses herself from the game saying she's got to film something real quick. Pop-Pop is suspicious and angry. Nana gets excited and starts eating cookies frantically. She turns to the camera Tyler has placed on the table and screams "YAHTZEE!"
Becca goes down to the basement, explaining to the viewer that she thinks her real grandparents have been trapped down there, and that's why Pop-Pop told them to stay away. She begins calling out for the real Nana and Pop-Pop but doesn't hear a response. In the corner, she sees a dumpster and hurries over to it. Inside are family photos of her real grandparents. She also sees something from Meadow Shade which she now learns is a MENTAL HOSPITAL. She digs some more and finds a hammer with blood and white hair on it and then sees..... THE CORPSES OF AN OLD WOMAN AND OLD MAN! Immediately behind her, Pop-Pop has appeared. He explains that he and the woman they know as Nana were mental patients and their real grandparents were volunteers. When they told them about their upcoming visit with their grandchildren, the two imposters decided it would be fun to experience in their place. But he is now determined to kill Becca. He chases Becca up into her room and locks her in. But she manages to defend herself, then busts the lock and escapes.
Its past 9:30 PM. Nana is beginning to "sundown" and starts crawling around the couches, chasing Becca. Meanwhile, Pop-Pop comes down to the kitchen with Tyler, who is frozen in fear, just like during the peewee game. Pop-Pop tells him he's under a spell and tells Tyler he never liked him. He goes behind the kitchen counter and removes his pants while the frozen Tyler looks on. Simultaneously, Becca continues to be chased by Nana. Becca's hiding in the corner facing the mirror but as normal, she doesn't look at herself, so she's oblivious that Nana's creeping up on her. Nana smashes Becca's face into the mirror and pieces of glass shatter all around them. Becca picks up a shard of glass as Nana jumps on top of her, clawing at her. ' In the kitchen, Pop-Pop has now revealed that he's removed a dirty diaper. He comments that he's noticed Tyler doesn't like germs and then shoves the dirty adult diaper into Tyler's face.
Meanwhile, Nana is on top of Becca, trying to kill her, but Becca stabs Nana to death with the glass shared. In the kitchen, Becca encourages Tyler to snap out of his frozen state, and he does, charging at Pop-Pop again and again and shouting as if he's tackling the big player on the peewee league. He has so much adrenaline that he pummels Pop-Pop to the ground and then smashes the refrigerator door against his head several times (unseen to the audience).
The kids run outside to find their mom and police cars out front. They hug their mom as the old time music that Becca promised to play at an important moment in her film plays.
Back home, the mom tells Becca that she used to be a great singer, and she could tell her mom was proud of her when she'd sing around the house as a kid. The fight happened because they didn't approve of her husband and when her mom blocked the door to keep her from leaving, she hit her mom and in response, her dad hit her. Stunned by the event, she stormed out and even though her parents tried to reconnect with her, she never talked to them again. She tells Becca not to hold on to anger. In response, we see the slideshow of Becca's dad that she previously said was banned from her documentary, played in full.
As the credits roll, we see Becca brushing her hair while looking at herself in the mirror while Tyler performs a rap to camera about the events that took place over those five days, including getting a used adult diaper shoved in his face and how it took two bars of soap to feel clean again. He says it did not taste like chicken.