In 1987, Jack Campbell (Nicolas Cage) and Kate Reynolds (Tea Leoni) are at the airport saying goodbye as Jack is about to board a flight to London for a one-year internship. Kate tells Jack that she has a bad feeling about him leaving and that she loves him deeply; she asks him to stay instead. Jack tells Kate that he loves her too and that one year apart is not gonna change that before getting on the plane.13 years later, Jack Campbell has become a single, wealthy Wall Street executive living the high life in New York City. He is in the midst of putting together a very important merger worth about $130 billion and has ordered an emergency meeting on Christmas day to ensure its success, much to the annoyance of some of his staff. Jack claims that once the deal is done, they will all be so rich they won't care about having missed Christmas to make it happen. Family clearly has no meaning to him.In his office on Christmas Eve, he is surprised to hear that his former girlfriend, Kate, has 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 worth $238. The store clerk believes that Cash is lying and refuses to validate the ticket. Cash pulls out a gun and is about to shoot the clerk before Jack steps in and offers to buy the ticket from him for $200, thus averting disaster. He and Cash settle their business deal outside and Jack assumes Cash needs help both mentally and financially. Cash replies that Jack may be the one who needs help, which Jack arrogantly refutes, saying he doesn't need anything. Cash leaves, cryptically telling Jack to remember that he brought this on himself. Jack goes back to his penthouse and falls asleep.The next morning, on Christmas Day, Jack wakes up in a suburban New Jersey bedroom with Kate next to him and two children. Shocked and confused, he drives their minivan back to his condo where the doorman and his closest neighbor don't recognize him and refuse to let him in. He then goes to his office where the doorman doesn't recognize him either. Jack points to a board that usually identifies him as the company president but is shocked to find his name is missing. 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 for him to figure out something, although he claims he doesn't know what the glimpse is about and what Jack is supposed to figure out. He then gives him a bicycle bell and leaves him on he sidewalk.Jack returns "home" where he realizes that he is living the life he could have had if he had chosen to stay with Kate instead of choosing his career. Instead, he has a modest family life where he is a tire salesman and Kate is a non-profit lawyer. The wealthy Jack doesn't understand the value of this life. Jack struggles to fit into the role of a family man, making many serious blunders such as missing Christmas morning with the family, heavily flirting with a married woman, not knowing how to love his wife and forgetting his wedding anniversary. However, Jack finally catches a break when his young daughter, Annie, observes that he may look like her father but he isn't her father. She assumes he is an alien who took her father's place for a while. After making him promise not to hurt the family, she starts assisting him with routine matters such as changing diapers, where the schools are, where he works and what he usually does every day. He begins to succeed in his life, bonding with his children, falling in love with his wife and working hard at his dull job.When his former boss and mentor, Peter Lassiter, happens by the store for a tire change, Jack can't help but comment about the merger his company has just brokered -- for a price less than what Jack himself would have aimed for. The chairman is impressed by his business savvy and ideas and invites Jack to a meeting with him and the president of the company. The president turns out to be Alan Mintz, one of Jack's sycophantic employees who now displays an assertiveness he did not have as a subordinate. Jack uses his knowledge of both men to prove that he is good at sizing people up and that he could be a good asset to the company. Lassiter offers him the job.While she is impressed by the potential salary increase and other benefits, Kate argues that they are very happy where they are and that they should be thankful for the life they have. She also argues that Jack's new career would either take him away from his children or force them to leave a home and a school they already love.Later, back at home, Jack finds his plane ticket to London from 1987 and discovers he actually took the flight, but then came back the next day to be with Kate. Aware of the choice he made back then, Kate tells Jack that if he truly needs to try this investment thing, she will follow him, move the family back to the big city and do what needs to be done just as Jack did for her back then.The next morning, Kate awakens to find Jack outside playing with their daughter, implying Jack has chosen to stay. Annie looks at Jack and finally recognizes that her true father has returned, to which Jack replies that he truly loves her.That night, while Jack is looking for rock salt in his garage, Annie rings the bicycle bell, which suddenly worries Jack. He goes to a local store where he finds the store clerk is Cash. Jack demands to stay in this life, but, though sorrowful of Jack's situation, Cash informs him there's nothing he can do. A glimpse is not meant to be permanent. Jack then goes home and asks Kate to promise him twice that she will always remember him the way he is right now. He then kisses his children goodnight and spends the night watching over his sleeping wife while he, himself, unsuccessfully fights off sleep.Jack awakens in his penthouse bed on Christmas Day. His experiences have caused him to realize he lives a lonely, unfulfilled life. He goes to work without much motivation to find out his $130 billion deal is at risk and he needs to move fast to save it. Jack offers to sacrifice his own Christmas to save the deal and reassures his team, but deep down, he doesn't care much any more.Following up on Kate's message to him the previous day, he finds her moving out of a luxury townhouse. Like Jack, she also focused on her career and became a wealthy lawyer. Furthermore, she only called him to give him back some of his old possessions before she moves to Paris. Jack invites her to go grab a coffee, but she's in a bit of a rush. She says maybe they'll have that coffee later in Paris one day. Jack goes home with his possessions, among which he finds pictures of himself and Kate, young and in love. Realizing he still loves her, Jack forgoes his very important deal and runs to the airport to intercept Kate before she leaves. He catches her and desperately describes the family they had in the alternate universe and the love they shared in an effort to make her stay. Shocked but intrigued, she agrees to a cup of coffee at the airport, suggesting that they might have a future after all.