Opening in the year 1989, inside the Le Domas mansion, siblings Daniel (Etienne Kellici) and Alex (Chase Churchill) are running around while being followed by a man. Daniel hides Alex in a closet before he is approached by the man, Charles (Andrew Anthony), the new husband of the boys' Aunt Helene (Elana Dunkelman). Charles pleads with Daniel to help him, as he has just been wounded and is being hunted. Instead, Daniel alerts the rest of his family to Charles's presence. Helene tries to save her husband, but the family drags Charles into a room to his grisly fate.
30 years later
Alex (now played by Mark O'Brien) is getting married to a woman named Grace (Samara Weaving). She is nervous but eager to join his family's gaming "dominion", which is how they came about their enormous wealth. The couple go for a photo-shoot prior to the ceremony, along with other members of Alex's family. Daniel (now played by Adam Brody) is joined by his wife Charity (Elyse Levesque), who is open about the fact that she only married Daniel for his family's money. Their father Tony (Henry Czerny) is visibly disapproving of Grace, but not nearly as much as Helene (now played by Nicky Guadagni), who shoots Grace a death glare. More approving is Alex and Daniel's mother Becky (Andie MacDowell), who tells Grace that only Alex's opinion matters. Moments later, everyone gathers for the ceremony, and Grace and Alex are married.
Later, Grace and Alex are trying to get intimate, but they are interrupted by Helene, who reminds Alex that he needs to join the family for something. Alex then explains to Grace that as per tradition with every new addition to the Le Domas family, she has to join the family for a game at midnight.
Grace and Alex reconvene with the family, who are now joined by Alex and Daniel's coke-head sister Emilie (Melanie Scrofano), her husband Fitch Bradley (Kristian Bruun), and their sons Georgie (Liam MacDonald) and Gabe (Ethan Tavares). Everyone gathers around a table while one of the maids, Clara (Hanneke Talbot), is reading the boys a bedtime story. Tony explains to Grace that this tradition was started by his great grandfather Victor, who made a deal with a mysterious benefactor known as Mr. Le Bail. He granted Victor and his future generations their wealth, as well as a special box, while Victor was told to give his own part in the bargain. The family passes Grace the box to draw a card from it and pick the game. Charity said she got Chess, while Fitch got Old Maid. Grace draws Hide and Seek, and the room falls silent.
Tony instructs Grace to stay hidden until dawn in order to win, which does not sound appealing to her at all. As she goes to hide, the family arms themselves with guns, except for Helene and Fitch, who get an ax and crossbow, respectively. Grace goes to hide in a dumbwaiter, and Alex chooses to sit this out. As the family goes off to find Grace, Alex leaves his room to go warn her, which Charity finds out about. Grace leaves the dumbwaiter and wanders before she is found by Alex, who covers her mouth and tells her to be quiet. Clara is walking around, as Georgie has gotten out of bed. As she looks for him, a gunshot rings out, and Clara's face is blown off. Emilie fired the shot, thinking it was Grace, but she realizes her error. Tony, Helene, Daniel, and Becky come in to help move Clara's body. Grace sits in horror at what she just witnessed. Alex pulls her into another room to explain the truth to her. Hide and Seek, for the Le Domases, means that they think they need to hunt Grace and kill her before dawn, or else they will die. Alex admits that he didn't tell her about this because he feared that she would leave him, but he promises to find a way to get her out safely.
Grace trades her heels for tennis shoes and tears off the bottom of her dress to move around more easily. She starts to walk out but is immediately spotted by Tony, Helene, and Daniel. They try to incapacitate her, but Emilie, fresh off another line of coke, starts wildly firing at Grace and misses. She runs for it, and the others realize that Grace knows the truth now. She is also found by Daniel, but he could not care less about the hunt, so he gives Grace a head start before alerting everyone.
The family gathers together to come up with a new plan, when another maid, Tina (Celine Tsai), comes in trying to tell Tony that she saw Grace. Emilie, who was given Fitch's crossbow, accidentally fires an arrow through Tina's head. Helene chops Tina's head off when she won't stop groaning. She then suggests to Tony that they turn on the cameras to locate Grace, but Tony is against it since it's not part of the tradition.
Grace finds a rifle and ammo, and she loads up. She hides in the kitchen while Alex tries to turn on the cameras and disable the locks on the doors and windows. The butler, Stevens (John Ralston), enters the kitchen to make tea. He hears Grace loading her rifle, and she is soon spotted. She tries to shoot Stevens, but he tells her the ammo is just for display. This doesn't stop her from smashing the teapot over his face and managing to evade him. Alex succeeds in unlocking the doors and windows, and Grace manages to get outside. Tony and Daniel find Alex in the control room, and Tony attacks him. Alex fights back but is talked down by Daniel, which only gives Tony a chance to knock Alex out. They handcuff Alex in his bedroom to prevent him from helping Grace. Meanwhile, she is outside on the roof but can overhear Tony and Helene talking. They know Alex has always rejected their tradition, being the "good son", but Helene thinks that Alex will come to accept his place in the family like she did when Charles was killed. She tried to fight it at first, but then realized she should have killed him herself, and she believes Alex will eventually follow in his family's footsteps.
Grace is almost out of the house when she is spotted by the third maid, Dora (Daniela Barbosa), who is hiding in the dumbwaiter. She tries to call out to the family now that she has found Grace, but Dora then accidentally shuts the dumbwaiter, causing her to be crushed in it. Grace then flees once again.
Grace makes it outside and runs to the stable. She sees Georgie walking there and thinks he will help her, but he draws a gun on her and shoots a hole through her hand. In response, she punches him in the face and knocks him out cold. Grace is then startled by a goat, leading her to fall into the Goat Pit, where the corpses of goats and unlucky family members are disposed. Grace climbs up to the top as the ladder starts breaking under her feet. She reaches the top but has a nail go through the hole in her hand. She manages to muster up the will to pull herself out, and then use part of her dress as a bandage. She runs back outside and manages to break through the front gate. She tries to flag someone down, but he is a prick and drives away from her. Not long after, she is found by Stevens, who tries to capture her. Grace fights him and attempts to strangle him with a piece of her dress. She takes his car, but doesn't see that he is still alive.
Becky orders Daniel and Emilie to dispose of the maids in the Goat Pit, since the others think they have been doing more harm than good to the hunt. Daniel is fed up with the family, but Emilie is adamant in sticking to the tradition so her boys won't be hurt.
Grace drives Stevens's car away and talks to a customer service agent, Justin (Nat Faxon), to get help. Unfortunately, since the car was already reported stolen, Justin shuts the car down, which leads to Stevens finding Grace and tranquilizing her. He drives back toward the mansion while video-chatting with the family to let them know he has Grace. He starts to play classical music over the radio and doesn't see Grace waking up, but the family does. They try to warn Stevens, but he doesn't see Grace as she kicks him twice in the head, causing him to roll the car over into a field. Stevens is dead, but Grace survives. She gets out and is found by Daniel. He says he can't let his family down, so he knocks her out and takes her back to the mansion.
Becky goes to Alex's room to try and talk to him. She thinks that while he loves Grace, he will ultimately stick to his family's wishes. Alex tells her that he will kill her if anything happens to Grace.
The family places Grace on an altar to begin the ritual to sacrifice her to Mr. Le Bail. They pass around a chalice and drink from it, but then they start violently puking just before Tony kills Grace. Daniel gave everyone something to make them sick, and he frees Grace from the altar. He tries to help her get out, but they are found by Charity. Knowing that he is no longer on their side, she shoots Daniel in the throat. Grace fights Charity and takes the gun from her. The bullets are gone, so she pistol-whips Charity. Grace tries to help Daniel, but he tells her to go. Alex breaks free from his cuffs and finds Daniel dying. He tries to save his brother, but it is too late. Tony then goes for Grace and tries to kill her himself, but she hits him over the head with a lantern and drops it, causing the curtains to catch fire. Grace is then found by Becky, who misses with her arrow but then tries to strangle Grace. Grace pulls the tablecloth down and grabs Mr. Le Bail's box, and she knocks Becky off of her before bludgeoning her to death with the box. Alex then comes across the scene and tries talking to Grace. He realizes that she will never stay with him now, so he turns on her and gives her up to his family.
Everyone continues with the ritual by holding Grace down themselves. Just as Alex is about to plunge the dagger into his wife, Grace breaks Helene's grip by clawing her hand, before letting the dagger go into her shoulder. She gets off the table and holds the dagger toward the family while screaming. They then see that the sun has come up, meaning it is now too late. As they recoil in horror for their impending death, nothing happens. However, Helene plans to kill Grace anyway. She grabs the ax, when suddenly...BOOM! Helene explodes into a bloody mess. The family realizes that they really are screwed. Fitch explodes next, followed by a begging Charity. Emilie tries to get away with Georgie and Gabe, but they all burst too. Tony tries yelling about how he followed the rules, but he turns into paste as well. Alex then tries to weasel his way into getting Grace's forgiveness, but she hands him her ring and says she wants a divorce. BOOM goes Alex. The fire from the curtains then spreads around the house, and for a brief moment, Grace sees the spirit of Mr. Le Bail sitting in his chair, and he nods approvingly to her.
Grace exits the burning mansion, now covered in blood. She sits on the steps to smoke a cigarette, just as police and paramedics arrive. An officer asks Grace what the hell happened, to which she simply replies, "In-laws."