Plot
Former cellist Claire Spencer and her husband, Norman, a scientist and professor, live at their lakeside home in Vermont. Their relationship is strained, particularly after Claire's daughter and Norman's stepdaughter, Caitlin, leaves for college. Adding to the tension, it has been a year since Claire suffered a serious car accident.
Claire observes that their new neighbors, Mary and Warren Feur, have a volatile relationship. When Mary disappears for several days, Claire suspects that Warren may have killed her. She starts experiencing unsettling occurrences: she believes she sees a woman's body in the lake, their front door opens on its own, and she senses an eerie presence. When a framed newspaper article about Norman falls from his desk and shatters, Claire discovers an unusual key. Soon after, she finds the bathtub mysteriously filled with water and sees a woman's reflection in it.
Norman is dismissive of Claire's fear that their house is haunted and suggests she see a therapist to deal with her mounting anxiety. During a session, Claire mentions seeing a woman who bears a striking resemblance to her. Claire and her friend Jody hold a séance. Shortly after, she again finds the bathtub filled with water. A message reading "You're next" briefly, then "you know" appears on the foggy bathroom mirror, and her computer types the initials "MEF." Convinced that Mary's ghost is haunting her, Claire confronts Warren. Mary appears next to him, alive and well, shocking Claire.
At a party, a friend reminds Claire of an argument she had with Norman a year earlier and asks if she is okay. When Claire returns home, the newspaper clipping falls again, and she notices a story on the back about a missing woman named Madison Elizabeth Frank, who bears a striking resemblance to her. Claire visits Madison's mother and secretly takes a lock of her hair.
Claire attempts to summon Madison's ghost. She becomes possessed by Madison's vengeful spirit and aggressively seduces Norman, shocking him by speaking in Madison's voice. The possession ends when she drops Madison's lock of hair. This triggers a repressed memory: Norman had an affair with Madison, one of his students, during a rough patch in their marriage. Claire had forgotten about it due to her car accident.
Shaken, Claire spends the night at Jody's house. Jody confesses that she once saw Norman arguing with a woman at a café in the nearby town of Adamant a year earlier. Returning home, Claire finds Norman unconscious in the bathtub, an electric hairdryer nearby. He assures her it was an accident. He admits that Madison had confronted him when he ended their affair but insists he did not kill her. Later, standing by the lake, Claire is pulled into the water by an unseen force. While submerged, she sees a jewelry box matching the necklace Madison wore in the photo. Norman pulls her out of the water, and they burn Madison's lock of hair.
Though she and Norman seem to reconcile, Claire's suspicions return when she suggests visiting Adamant, and Norman pretends not to know the town. Claire visits Adamant alone and recognizes the same necklace and jewelry box in a shop window. She recovers the box from the lake and unlocks it with the key from Norman's office, finding Madison's necklace inside. When she confronts Norman, he claims that Madison killed herself and that, in a panic, he pushed her car into the lake with her body inside. He calls the police, per her insistence. While he is getting ready, she discovers that he faked the call. Suddenly, he appears and attacks Claire, paralyzing her with halothane. Norman confesses that he murdered Madison when she threatened to expose their affair to the dean. He mocks Claire, saying he initially assumed she made up the ghost story to subconsciously reveal the truth but then realized she actually believed it. He places Claire in the bathtub, filling it with water to stage her suicide. As he removes Madison's necklace from Claire's neck, her face briefly transforms into Madison's corpse. Startled, Norman stumbles backward, smashing his head against the sink and knocking himself out.
As the drug begins to wear off, Claire unplugs the drain and escapes drowning. As she tries to grab the phone, Norman regains consciousness and chases her. Claire flees in his truck with their boat attached, but Norman climbs inside and attacks her, causing the vehicle to crash into the lake. Trapped underwater, Norman tries to drown Claire. Suddenly, the wreck of Madison's car submerged beneath them shifts. A part of the boat impales the roof, dislodging Madison's decayed corpse. Madison's body floats up and appears to come to life, grabbing him. Claire escapes while Norman drowns. Madison's corpse returns to its lifeless state, drifting into the depths as her necklace slips from Norman's hand.
That winter, Claire visits a cemetery and places a red rose on Madison's gravestone.
