Jack Campbell is a single, wealthy Wall Street arbitrageur living the high life in New York City. He is surprised to see that his former girlfriend, Kate, tried to give him a call after many years, and wonders (consulting his mentor at the firm) whether to answer it the next day. That night, around Christmas, he walks into a convenience store and convinces an angry lottery contestant, known as "Cash", not to shoot the clerk. He offers to help Cash before going to sleep in his penthouse.

The next morning, on Christmas, he wakes up in a suburban New Jersey bedroom with Kate and two children. He hurries back to his office and condo, but not even his closest friends recognize him. He meets Cash, who is driving the Ferrari that Jack lost track of, and begins to explain to Jack that he is in an alternate universe in order to learn a lesson.

Jack realizes that he is living the life he could have had if he had not gone to London and become an investment banker but had stayed in the United States with his 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 alternate life. There he meets his old mentor, who once again gladly offers him a job, and a former sycophant employee who 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 complementary extreme luxuries, Kate convinces him to decline and spend time with his family.

Just as Jack is finally realizing the true value of his new life, his epiphany jolts him back to his wealthy —yet as he now realizes, lonely— former life. In desperation, he forgoes closing his $130 billion pharmaceutical acquisition deal to intercept Kate (who had left the message yesterday). He finds her moving out of a luxury townhouse - like Jack, she was also left to focus on her career and became a very wealthy lawyer, and only called him to give back some of his old possessions. Before she moves her business to Paris, he runs after her at the airport and describes the family they had in the alternate universe in an effort to get back together with her. Shocked but intrigued, she agrees to a cup of coffee at the airport, suggesting that they might have a future after all.
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