Jack Campbell is a single, wealthy Wall Street Executive living the high life in New York City. He is in the midst of putting together a billion dollar merger and has ordered an emergency meeting on Christmas day to ensure its success. Family clearly has no meaning for him. In his office on Christmas Eve, he is surprised to hear that his former girlfriend, Kate, tried to call him after many years. After reminiscing a bit, he walks into a convenience store where a lottery contestant, Cash, barges in saying that he has a winning ticket. The store clerk believes that Cash is lying and refuses to give him his winnings. Cash pulls out a gun and is about to shoot the clerk before Jack offers to buy the ticket from him, thus averting disaster. He and Cash settle their business deal outside and he arrogantly offers to help Cash before going to sleep in his penthouse.
The next morning, on Christmas Day, Jack wakes up in a suburban New Jersey bedroom with Kate and two children. Shocked and confused, he hurries back to his office and condo in New York, but not even his closest friends recognize him. Confused, Jack runs out to the street and encounters Cash, who is now driving Jack's Ferrari. Cash explains that Jack is experiencing a glimpse of an alternate universe in order to learn a lesson. What that lesson is unknown and is personal to Jack's life.
Jack realizes that he is living the life he could have had if he had not gone to London to study and become an investment banker but had stayed in the United States with his then girlfriend. He instead has a modest family life, where he is a tire salesman and Kate is a non-profit lawyer. Jack struggles to fit into the role of a family man, making many serious blunders such as missing opening Christmas presents, flirting with a married woman and forgetting his anniversary. Very soon, Jack's young daughter realizes his secret and decides to assist him in surviving his new life. He begins to succeed in his life, bonding with his children, falling in love with his wife and working hard at his dull job.
He suddenly finds himself being offered a contract to work at the very same investment firm from his real life, having impressed the Chairman of the firm with his business savvy when he came in for a tire change. His old mentor once again gladly offers him a job, while a formerly sycophantic employee is instead in Jack's old position, with an assertiveness he did not possess as a subordinate. While he is wowed by the potential salary and other complimentary extreme luxuries, Kate argues that they are very happy where they are and that they should be thankful for the life they have.
Just as Jack is finally realizing the true value of his new life, he sees Cash again (now a store clerk) and demands to stay in this life, but, though sorrowful of Jack's situation, Cash informs him there's nothing he can do. So, his epiphany jolts him back to his wealthy yet as he now realizes, lonely and unfulfilled former life on Christmas day. In desperation, he forgoes closing his $130 billion pharmaceutical acquisition deal to intercept Kate (who had left the message the day before). He finds her moving out of a luxury townhouse. Like Jack, she also focused on her career and became a very wealthy lawyer. Furthermore, she had only called him to give back some of his old possessions. Before she moves to Paris, he runs after her at the airport and describes the family they had in the alternate universe in an effort to win back her love. Shocked but intrigued, she agrees to a cup of coffee at the airport, suggesting that they might have a future after all.