Plot

In the town of Thayer, located in New England, the film focuses on Everett Stone (Dermot Mulroney) and his incredibly close family. Meredith (Sarah Jessica Parker), Everett's anxious, bumbling and tightly wound girlfriend, is dreading spending the Christmas holidays with Everett's family. Meredith is an uptight, contemporary New York City career woman from Bedford, New York.

The family includes mother Sybil (Diane Keaton) the family's strong-willed & bohemian matriarch, father Kelly (Craig T. Nelson) a college professor in his sixties, brother Ben (Luke Wilson) a stoner and film editor, living in Berkeley, California, elder daughter Susannah (Elizabeth Reaser), youngest son Thad (Tyrone Giordano) a deaf and gay architect, who lives in Boston and is contemplating adopting a child with his African American partner Patrick (Brian J. White), youngest daughter Amy (Rachel McAdams) a schoolteacher in the Pioneer Valley, pursuing her master's degree at UMass Amherst.

Susannah is a stay-at-home mom who lives in suburban Chicago and has one child, Elizabeth (Savannah Stehlin), she is expecting her second. Amy had previously met Meredith and took an immediate dislike to her.

Everett's tight-knit family responds awkwardly, and soon sternly, to Meredith's stiffness, making her feel like an outsider. Ben, Everett's brother, is the only one who seems to like Meredith. The holiday gathering starts with a series of embarrassing events for Meredith. Meredith talks loudly to Thad, not knowing that he could read lips. She shakes hands with Kelly when he wants to hug her. She gets pushed out from the family picture. Meredith feels awkward about sleeping in the same room as Everett, in his parent's house and insists on a separate room, but that means she pushes Amy out of her room.

Meredith only opens up when she tells a long, boring story of how she and Everett met during a business trip. Meredith comes across as controlling and dominating. At dumb charades, Amy picks a movie for her "The Bride Wore Black", and then accuses Meredith of pointing at Patrick during the game, as a clue for the 4th word.

Meredith confronts Everett and says that even he has started to see her differently due to the influence of his family. The next morning, Meredith confronts Amy and wants to know what she did wrong. Amy refuses to engage and taunts Meredith that she cares about whether Amy likes her or not.

Meredith opts to stay at the local inn. Everett confronts his family about their deliberate rough treatment of Meredith and warns them to treat her fairly. After they leave, the family has a meeting where Kelly asks everyone to be civilized to Meredith. But Amy and Ben are clear that Everett and Meredith cannot marry. Ben says that they don't even love each other. Meanwhile Everett returns and asks Sybil for the family heirloom, the grandmother's wedding ring, as he wants to propose to Meredith. Sybil refuses as she cannot see Meredith as her daughter-in-law. Everett goes shopping for a ring with Thad, and even Thad asks Everett not to marry Meredith.

Meredith returns to the Stone home to prepare for a Christmas breakfast, saying that she wants to contribute to the holidays. But she did not know the allergies of the family members and then had to redo her dishes.

Meredith begs her sister Julie (Claire Danes) to take a bus down to Thayer and join her for support. Everett finds himself drawn to the friendly, more outgoing Julie, whom his family receives very warmly after Julie has a fall while getting off the bus. Julie works at a foundation awarding grants to artists.

Meredith desperately tries to fit in with the Stones, but her strained attempts prove disastrous. During dinner, Everett's gay, deaf brother Thad and his partner Patrick express their plans to adopt a child, prompting a discussion about nature versus nurture and sexual orientation. When Meredith clumsily attempts to engage in the conversation, her choice of words offends everyone and Everett's father Kelly, the most understanding of the family, angrily shuts her down.

Distraught, Meredith attempts to drive off but crashes Everett's car, and Ben comes to comfort her. Ben's attraction to Meredith is apparent and the two of them end up at a local bar where, after several drinks, Meredith begins to relax and enjoy herself. She invites local paramedic Brad Stevenson (Paul Schneider), the high school flame of Everett's sister Amy, to the Stones' house for Christmas breakfast. The next morning, she awakens in Ben's bed and incorrectly assumes that they had sex.

On Christmas Day, the Stone children learn that Sybil, their mother, and a breast cancer survivor, recently developed an aggressive recurrence of the disease. Sybil, who originally refused Everett's request for his grandmother's ring to propose to Meredith, reconsiders her position and offers it to him. But, by now, his feelings for Meredith have shifted to her sister Julie. In a moment of emotional confusion or clarity, he asks Julie to try on the ring, and it gets stuck on her finger. When Julie and Meredith lock themselves in the bathroom to get the ring off, they assume Everett is about to propose to Meredith.

The family exchanges gifts and Meredith, unaware of Sybil's failing health, presents each family member with a framed, enlarged photograph of Sybil taken when she was pregnant with Amy, thinking it's Sybil and Everett. Everyone is touched by her gesture, and Meredith relaxes slightly. But, when Everett asks to talk to her, she blurts out that she will not marry him. He counters that he didn't plan to ask her, and Meredith emotionally breaks down in front of the family. All the personality conflicts come to a head, and everyone begins the process of healing.

One year later, the family reunites again for Christmas. Meredith and Ben are a couple, as are Everett and Julie, and Amy and Brad. Thad and Patrick have adopted a baby boy named Gus, and Susannah, the oldest daughter, has had another baby boy named Johnny. It is implied that Sybil passed away over the previous year, and the family remembers her as they gather around the Christmas tree.