Years ago, young brothers Daniel and Alex Le Domas watch in terror as a wounded man named Charles begs for help inside their family's estate. The Le Domas family hunts Charles down, drags him into a secret room, and murders him with shotguns and axes. Thirty years later, Alex Le Domas returns to his family estate to marry Grace, a warm-hearted young woman who grew up in the foster care system and yearns to belong to a real family. Alex's family made a global fortune in board games and sports teams. Following the wedding, Alex warns Grace that the family has an eccentric initiation tradition: at midnight on her wedding night, Grace must draw a card from a mysterious antique puzzle box gifted to their ancestor Victor Le Domas by a merchant named Mr. Le Bail. Grace draws the rare card: "Hide and Seek." Grace assumes it is a childish parlor game and runs off to hide in a service dumbwaiter. Downstairs, family patriarch Tony Le Domas explains the stakes to the household. The family made a demonic pact with Mr. Le Bail: in exchange for immense wealth, any bride or groom who draws Hide and Seek must be hunted down and sacrificed before sunrise, or the entire family will perish. Armed with antique weapons, the family begins the hunt. The cocaine-addicted sister Emilie accidentally shoots and kills an innocent maid with a shotgun. Grace witnesses the murder and panics. Alex finds Grace and confesses the truth about his family's dark pact. Desperate to save her, Alex disables the mansion's electronic security system so Grace can escape. Grace rips off the bulky train of her wedding dress, puts on yellow sneakers, and fights for her life. As she flees through the house, Emilie accidentally kills a second maid with a crossbow. Young Georgie shoots Grace through the hand with an arrow, but Grace punches the boy and escapes into the courtyard. Grace falls into a deep outdoor pit filled with rotting goat carcasses and skeletal human remains from past sacrifices. In an agonizing scene, Grace pushes her impaled hand through a rusty nail on a wooden ladder to climb out of the pit. She reaches the road, but the family butler Stevens ambushes her. Grace fights Stevens in his car, causing a crash that knocks Stevens out, but she is recaptured by the family and dragged back to the estate. Tied to the ritual table in the dining room, Grace is prepared for the dawn sacrifice. Daniel, disgusted by his family's cruelty, secretly spikes the sacrificial chalice with a sedative. When the family drinks, they begin vomiting blood, giving Daniel time to free Grace. Daniel's cold-hearted wife Charity shoots Daniel in the throat, and Grace bludgeons Becky to death with the heavy puzzle box. Seeing his brother dead and his mother killed, Alex snaps. Believing Grace could never love him after this horror, Alex betrays Grace and pins her to the floor as Tony prepares to plunge a dagger into her heart.

Just as Tony raises the sacrificial knife above Grace's chest, a beam of sunlight breaks through the stained-glass window. Sunrise has arrived. The ritual deadline has passed, and Grace is still alive. The family freezes in absolute terror. For several long seconds, nothing happens. Tony looks around, realizes he is still breathing, and bursts into relieved laughter: "We're alive! It was all a lie! It was just a superstition!" Suddenly, Aunt Helene gasps as her chest begins glowing bright red. Without warning, her entire body explodes like a giant balloon filled with blood, splattering guts all over the walls. One by one, every single member of the Le Domas family spontaneously explodes into wet chunks of gore: Tony, Emilie, Fitch, and Charity all burst into bloody mist. Alex is the last one left standing. Covered in his family's blood, Alex looks at Grace and desperately begs for forgiveness, asking if they can still work things out. Grace looks back at her husband with cold disgust, throws her wedding ring on the floor, and says: "I want a divorce." Alex instantly explodes into a cloud of red mist. As the room burns from overturned candles, Grace turns toward the doorway. Sitting comfortably in an armchair is the ghostly apparition of Mr. Le Bail, the devil himself. He tilts his head, tips his hat, and gives Grace a respectful smirk of approval before vanishing into thin air. Grace walks out onto the grand front porch of the burning mansion, covered head to toe in blood, wearing her torn wedding dress. She sits down on the stone steps and lights a cigarette. A police car pulls up with sirens flashing. The officer steps out, looks at the smoking ruins and the blood-soaked bride, and asks: "What happened to you?" Grace takes a slow drag from her cigarette and calmly replies: "In-laws."