Paltrow and Schaech play a dating couple, living together in a New York City apartment. At the film's beginning, the two are driving towards the Kentucky farmhouse that Jackson (played by Schaech) grew up on, primarily to introduce Helen (Paltrow) to Jackson's mother, Martha (played by Lange). The farmhouse and the land around it is known as Kilronan. The week that the couple share with Martha proves a success, and the dating couple heads back to New York.
Soon, however, Helen discovers that she is pregnant. When she informs Jackson of this, he is ecstatic, and asks Helen to marry him. She agrees, and the two inform Martha, who immediately guesses that Helen is pregnant. Jackson and Helen have a wedding at Kilronan, at which Helen first makes the acquaintance of Jackson's grandmother, Alice. Alice is not Martha's mother, but that of Jackson's deceased father. After making a few comments about how difficult it is to trust Martha, Alice is interrupted by Martha herself, and the conversation is cut short.
Upon returning home to her New York apartment one night, Helen is assaulted by a stranger, who steals her precious locket. When Helen reveals she's pregnant, he cuts her abdomen, warning her with a taunt: "I know where you live." After a frantic check, it is discovered that no damage has been done to the child.
As Jackson and a still-apprehensive Helen return to the apartment, Helen is startled by Martha's "surprise!". At dinner, the subject matter of the conversation becomes very serious, as Martha hopes to sell Kilronan, as she cannot keep it running alone. Jackson is disappointed with the offer, as just two years earlier they were offered triple the amount. When Martha leaves, not having earned Jackson's permission to sell the land, an idea strikes Helen. She wants to move to Kentucky and in with Martha for a year and help renovate the land, making it much more valuable to a buyer. Jackson is hesitant.
The two discuss the matter the next day, and Jackson explains that his father died in that house. According to Jackson, his father had been cheating on his mother with "Robin Hayes," and his mother had stormed out. In an attempt to "save" Martha, a very young Jackson had run up the stairs, running into his father, who had subsequently fallen down the basement stairs. The fall had killed him. This convinced Helen that Jackson should return home, facing his "old ghosts."
Martha is quite overjoyed to have her son return home, and it soon becomes apparent that she is jealous of the attention he showers on Helen. She soon begins to divide the couple somewhat, explaining to Jackson that she needs time alone, while arousing suspicion in Helen that Jackson is being distant because he is cheating. It isn't long before Helen instead suspects Martha of being very jealous, and she commonly vents to Alice at her nursing home.
When Martha learns of Helen's trips to Alice, she is furious. Secretly, she pays a visit to Alice, warning her to stay away from her family. Despite this threat, Helen continues to visit Alice, even giving her a picture of the baby's sonogram. She soon learns that Jackson's story of his father's death isn't quite accurate, that news reports stated he landed on a nail puller at the bottom of the stairs in a freak way, crushing his sternum. Unsure of what this means, Helen decides to look into it.
Helen continues to grow tired of Martha's behavior, including Martha's spreading of a rumor that Helen insists on having the baby at home. She also begins to notice that Martha insists the baby must be a boy, and learns that Martha aborted her first pregnancy, a girl. She feels as though Martha is tearing her marriage apart, and she confronts Jackson about it. Reluctantly, he agrees that the two should return home to New York. While Martha is very upset, she agrees, but reminds Jackson that he has to take the farm's horses to the race tracks for the weekend. Once he returns, he and Helen will head to New York.
That evening, Martha bakes a cake for Helen, secretly laced with oxytocin, a potent labor inducer (she obtained the oxytocin from a stable, where it is used to induce horses). Upon Martha's insistence, Helen reluctantly agrees to eat the cake, and when she rouses the next morning, she feels strange. With Martha nowhere to be found, she searches for her, spotting a flicker of light in a barn. When she finds its source, however, she is shocked to find a baby room, as though Martha planned to raise the baby. She searches through one of the dressers and is horrified to find the locket that was stolen from her during the assault in New York. Martha finds her, claiming that the room is a surprise gift to Jackson and her, that she assembled for presentation. Helen pretends to buy into it and, on the way inside, requests water. When Martha goes to get it, Helen retreats, getting into her Ford Explorer. Unable to find a way out and with her truck getting stuck in mud, she soon crawls to the highway, where Martha finds her.
Soon, Helen is giving birth, with Martha sitting nearby. Completely without medicine, Helen is in pain, and Martha leaves the room to answer a phone call from Jackson. She tells him that everything is okay, but when Helen screams into the phone, Martha hangs up. Jackson then races from the track to hurry home.
Once the baby boy is delivered, Helen begs Martha to hand her the baby, but Martha ignores her, telling the baby that she is his mother. As Helen weeps, Martha returns to Helen's bedside with a syringe full of morphine. She begins to inject it in Helen's arm, with the intention of killing her, but Helen knocks the syringe away. When Martha retrieves it, she hears Jackson's footsteps. In a scramble, she quickly cleans up the scene, meeting Jackson at the door. She tells him to leave Helen alone, as he has no idea what she's been through. The two leave Helen in a peaceful slumber, with Martha introducing Jackson's son.
That night, Martha enters Helen's bedroom, the syringe again in her hands. Still trying to make it appear as though Helen died during the birth, she creeps toward the bed. She's shocked, however, to find that Jackson is sitting in a chair next to the bed, waiting vigilantly for Helen's awakening. Despite his mother's insistence that he return to bed, he stays committed to seeing her awaken. Martha leaves the room in fear, as her plan has been foiled. The next morning, Helen awakens to see Jackson. Knowing she has survived Martha's plot, she tells Jackson to ask Martha to make breakfast.
At breakfast, Helen enters the house with a mysterious object in her bag. Jackson is naturally curious about what it is, and its mystery makes Martha incredibly nervous. When Helen explains that it's one of the objects that killed Jackson's father, he is confused. She goes on to explain what had been in the newspapers, and that the precision was almost as sure as someone hitting him with one. Martha becomes enraged at this, going off on a tirade that it was not her. Helen then claims that no one is accusing her of murder, and that plenty of cheating women don't kill their husbands. She goes on to explain that Robin Hayes, the "woman" Martha had told Jackson his father had cheated on her with, was actually a male horse wrangler, with whom Martha had an affair. Martha is completely outraged as the truth is revealed. Helen then reveals a bruise she has from Martha's murder attempt, enraging Jackson. The two prepare to leave, with Jackson never wanting to see Martha again. He wants her out of Kilronan and out of their lives. In a last-ditch effort, Martha claims that Helen is only doing this because she is jealous. "She wants to be me," Martha laughs. Helen slaps her hard enough to knock her to the ground. Helen and Jackson leave Martha on the floor, sobbing.
The movie ends with the couple visiting Jackson's grandmother Alice, presenting her with her great-grandson.
