Brad (Vince Vaughn) and Kate (Reese Witherspoon) are an upscale San Francisco couple. Having both come from dysfunctional families, with divorced parents and siblings with out-of-control kids, the two disdain the idea of getting married or having kids. In an effort to avoid their families at Christmas, the two go on vacation abroad while pretending to be doing charity work there. The third Christmas of their relationship, Brad and Kate plan to go to Fiji, but are trapped at San Francisco International Airport by a fog bank that cancels every outbound flight, and interviewed by a news crew, alerting their families to the fact they're stuck at home for the holidays.
With no way to get out of it, Kate and Brad find themselves visiting their families on Christmas Day. They first visit Brad's father (Robert Duvall), then Kate's mother (Mary Steenburgen), then Brad's mother (Sissy Spacek) and, finally, Kate's father (Jon Voight), thereby celebrating four Christmases in one day. As they brace themselves for a marathon of homecomings, Brad and Kate expect the worst, but are nevertheless unable to prepare themselves enough for what they get. As the day progresses, each discover a new secret about their partner that they had previously been too embarrassed to share, namely that Brad's real name is "Orlando", and Kate has a fear of inflatable castles, stemming from being ostracized as a child, and these discoveries put an intense strain on their relationship. While Brad counts down the minutes to freedom, Kate finds herself looking at the lives of Brad's and her own siblings and comes to realize that she does want a marriage and children of her own, the prospect of which frightens Brad when she mentions it to him. Eventually, in the final visit of the day, at Kate's father's house, Kate asks Brad to let her spend the visit on her own and claims to her family that they had split up. Meanwhile, Brad spends some time at his own father's house with just his father and realises that he wants a marriage and children, and that he loves Kate way too much to leave her. He returns to her and they discuss the possibilities of having a child and getting married. The two then embark on their holiday in Fiji.
A year later on New Year's Day, the couple welcomes their first child, a baby girl. They have spent the last nine months hiding from their families. As their baby is the first born in the New Year, a news crew comes to congratulate them--once again revealing them, and their new baby, to the whole city, and their families.