Drew, played by Ben Affleck, impresses a group of advertising executives with an innovative promotional concept for spiked eggnog. However, when he surprises his girlfriend with two tickets to Fiji, she expected a marriage proposal instead and leaves him heartbroken. The following day, in an attempt to win her back, Drew instructs his secretary to send her a thoughtful gift, but foreshadowing a lonely Christmas, he soon discovers that none of his friends or family wish to host him for the festive season.
In a desperate move, he rushes to the airport to confront his girlfriend's therapist before her flight. The therapist gives Drew some surprising advice—to articulate his grievances on paper and burn them in a place that holds childhood memories. Thus, Drew finds himself outside his childhood home, only to realize it is now inhabited by another family. The Valcos, portrayed by James Gandolfini and Catherine O'Hara, are bickering over their son’s excessive computer use when they spot Drew lighting a fire in front of their house. Unsurprisingly, Mr. Valco reacts by hitting Drew on the head with a shovel.
Inside the Valco home, the family curiously wonders if Drew is alive. Upon regaining consciousness, Drew finds himself invited to dinner, a short-lived invitation that he soon turns into an outrageous proposal: he offers the family a staggering $250,000 to act as his family for the holiday season. Legal papers are signed, and hilariously absurd Christmas traditions follow, including an awkward tree shopping trip where Mr. Valco is compelled to don a Santa hat. When Drew's new sister, Alicia, played by Christina Applegate, arrives and resents the new arrangement, tensions rise.
The evening of their first festive dinner is filled with chaos when Drew introduces an elderly man as ‘Grandpa,’ a local actor he hired for the role. As the evening unfolds, the Valco family grapples with their own issues, with Mr. Valco revealing that he and his wife are considering separation—a revelation that complicates Drew’s holiday charade. In a bid to uplift the spirit, Drew encourages Mr. Valco to treat himself, while suggesting a photoshoot to boost Mrs. Valco's self-esteem.
As Drew builds connections with Alicia, moments of joy emerge, including sledding adventures where they escalate the excitement by taking a helicopter to a nearby mountain. Here, amidst laughter and potential romance, they share personal stories that expose their vulnerabilities. Drew's fabricated family situation intensifies as the real-world complications begin to weave into his elaborate tale, especially when his girlfriend wishes to reunite, demanding to meet his family.
With time running out, Drew resorts to increasing the family's compensation for their ongoing role-play. When the original 'Grandpa' cannot make it, his understudy replaces him, creating even more unpredictable tension. In the ensuing chaos, Drew finds himself torn between burgeoning feelings for Alicia and his obligations to his ex-girlfriend. The saga reaches a peak filled with misunderstandings, escalating frustrations, and unexpected reunions, leading to cringe-worthy moments that involve a botched family photo and a website exposing private moments of Mrs. Valco.
Ultimately, as the family faces this tumultuous experience together, true connections begin to surface, culminating in heartfelt conversations. After dealing with an unexpected fallout, an emotionally charged reunion occurs at a Christmas performance of "Scrooge," where Drew and Alicia share a kiss amidst the spectacle, finally coming full circle just as Drew’s empty apartment echoes the loneliness he feared to confront.
