NOTE: This spoiler was submitted by Jeremy

The film starts at a firm where co-workers Charlie (Dan Stevens) and Mina (Sheila Vand) are looking at a listing for a getaway home for the weekend. Joining them is Charlie’s brother/Mina’s boyfriend Josh (Jeremy Allen White), who is excited at the prospect of a mini-vacation. At home, Charlie discusses with his wife Michelle (Alison Brie) how Josh has been something like the black sheep in their family, but Mina has helped him get his act together.

The couples prepare to head off to the house, but Charlie is annoyed that Josh is bringing his and Mina’s dog Reggie with them, since the homeowner told them that pets aren’t allowed. Josh insists on taking Reggie anyway and plans to hide him. On the trip there, the couples discuss how Mina, who is of Middle Eastern descent, was denied the application for the house, while Charlie filled out the application an hour later and was instantly accepted.

The four arrive at the house and see that the front door is unlocked. They are startled when the homeowner, Taylor (Toby Huss), shows up. There is immediate tension when he appears to make condescending remarks toward Mina. After a tour of the place, Mina confronts Taylor over her application getting rejected potentially being over racial concerns, which Taylor denies but also refuses to get into it and leaves. Mina and Josh also find a door with a lockpad somewhere under the house.

The four go for a walk on the beach. Charlie and Mina walk ahead together while Josh and Michelle chat. He worries that Mina will leave him because he thinks she is too good for him, as well as the fact that she spends so much time with his brother. Michelle encourages him to not worry so much about it, even though Michelle is also slightly put off by Mina and Charlie hanging around as closely as they do. When they return to the house, they find that Taylor came back and left them with a telescope to look up at the stars, but Mina finds it creepy.

Later that night, Michelle takes out some drugs for them to have fun, but she starts to feel tired pretty quickly, so only Charlie, Josh, and Mina get high. Josh goes to bed while Charlie and Mina go to the hot tub. They start to talk, which turns into Charlie telling Mina how she makes him a better person. It leads to them making out before they realize they should stop. They then take it to the shower and have sex there, but are ultimately both still remorseful.

In the morning, Charlie and Mina are too hungover to go on their planned hike. Josh and Michelle go together, and Michelle brings up how Charlie was with another woman, Jen, before marrying her. Josh replies that he had a similar thing with a woman named Zoe around the time he started getting with Jen, which leads Michelle to worry. Meanwhile, Charlie and Mina say that their encounter was a one time thing. She goes to take a shower but is startled to find what looks like a camera in the showerhead. She shows Charlie, and they fear that their affair may have been recorded, but they can’t take it to the police or else Josh and Michelle will find out. Upon returning home, Michelle asks Charlie about Zoe and Jen, which he hesitantly admits to, and she acknowledges that he has a pattern. Charlie knows Josh told her this, but she is more annoyed that he was never upfront about things.

Michelle tries to shake things up by bringing out more drugs. Josh gets freaked out when he finds that Reggie isn’t where he was originally tied up to, fearing he may have ran off the cliff or that Taylor came by and took him. To make matters worse, Michelle gets too high and calls Taylor to fix the hot tub since the jets aren’t working. When he arrives, Josh asks Taylor if he took him, which he denies but also asks why they brought the dog out in the first place. After he fixes the jets, he is about to leave, but Mina goes to ask him about the camera in the showerhead. She shows it to him, and he says he didn’t put it there but will call the police for them. Mina tries to stop him, and Josh witnesses the struggle, thinking Taylor is attacking Mina. He grabs Taylor and punches him repeatedly in the tub before the other three have to pull him off Taylor, who is now unconscious. Everyone goes outside where Mina mentions the hidden camera. As they are talking, a masked figure enters the bathroom and smothers Taylor to death.

The four find Taylor’s body and think Josh killed him. Michelle panics and thinks they should call the police, but the others resolve to get rid of Taylor’s body and make it look like he got drunk and stumbled off the cliffside. Charlie, Josh, and Mina carry his body to the cliff and throw him over, but he lands on a rock not too far below. Josh climbs down carefully to push the body off into the ocean. Back at the house, the figure puts the video of Charlie and Mina’s affair on the TV, which Michelle witnesses. When the others get back, Charlie finds Michelle driving away with the car. She tells him she knows what he did and leaves. As she drives through the woods, Michelle drives across a spike strip and crashes into a tree. As she texts Charlie for help, the figure comes running to her window.

Charlie gets Michelle’s message and goes to help her. He runs into the woods and finds the car broken into. He calls her phone and sees it shining in the darkness. Charlie walks toward it, only to come across Michelle’s body before the figure comes to attack and kill him as well. Meanwhile, Josh receives a voicemail of Charlie and Mina discussing covering up the affair before being sent the shower video. Josh is angry and goes to confront Charlie, only for the masked figure to come back and kill Josh by driving a hammer into his head. Mina runs out of the house and tries to lose the figure in the woods, only to become disoriented in the fog and fall off the cliff to her death.

The masked man cleans up the house and removes any evidence of him being there, or the murders. It also turns out Reggie is alive, as he runs back into the house, but steps back when the man tries to pet him. The man is later seen purchasing an apartment.

Over the credits, we see surveillance footage of a new couple renting out the apartment with their kid. As the couple sleeps at night, the masked man emerges from their closet to attack.

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Two couples - Charlie and Michelle, Josh and Mina - rent out a vacation home for the weekend. While there, Charlie and Mina have sex while their significant others are sleeping. Mina later finds a camera in the shower where the affair took place, and they worry that the homeowner, Taylor, is spying on them. They also know that they cannot report it to the police without Josh and Michelle learning of their infidelity.

Michelle gets high and calls Taylor to fix the hot tub when the jets don't work. Josh, who brought his dog Reggie, becomes freaked out when Reggie goes missing, thinking that Taylor took him. He confronts Taylor about it but he denies it. After he fixes the hot tub, Mina shows him the camera and confronts him, leading to a struggle where Josh comes in and beats Taylor unconscious. The four leave the room, wherein a masked man smothers Taylor to death. The couples find the body and dispose of him over the cliffside to avoid implicating Josh. As this happens, the masked man plays the video of Charlie and Mina having sex for Michelle to see, and she leaves Charlie. The masked man causes her to crash her car in the woods and attacks her there, after she texts Charlie. Charlie finds Michelle and is killed as well. The man returns to the house and kills Josh before chasing Mina outside, where she becomes lost in the fog and falls over the cliff to her death.

The man removes evidence of foul play from the house and then purchases an apartment to rent out to another couple that he later stalks and attacks.