Tim Lake (Domhnall Gleeson) is a young man from Cornwall, England. He grows up in a house by the sea with his father (Bill Nighy), his mother (Lindsay Duncan), his absent-minded uncle (Richard Cordery), and his free-spirited sister, Katherine (Lydia Wilson), who is known to family and friends as Kit Kat. At the age of 21, Tim is told by his father that the men of his family have a special gift: the ability to travel in time. This supernatural ability is subject to one constraint: they can only travel to places and times they have been before. After his father discourages Tim from using his gift to acquire money or fame, he decides that he will use it to improve his love life.

The following summer, Kit Kat's friend Charlotte (Margot Robbie) comes to the house in order to spend her holiday with Tim's family. Tim is instantly attracted to her and at the end of her stay decides to tell her how he feels. She tells him that he should not have waited until the last day, that perhaps if he had told her earlier, something could have happened between them. Tim travels back in time and, the second time around, tells Charlotte in the middle of the holiday how he feels. In this instance, Charlotte uses the exact opposite excuse, saying that it would be better if they waited until the last day of the holiday, and then something could potentially happen between them. Heartbroken, Tim realizes that Charlotte is not attracted to him and that time travel will not make it possible for him to change her mind.

After the summer, Tim moves to London to pursue a career as a barrister. He is put up by his father's old acquaintance, Harry (Tom Hollander), a misanthropic playwright. After some months, Tim visits a Dans le Noir restaurant, where he meets Mary (Rachel McAdams), a young American woman who works for a publishing house. The two flirt in the darkness of the restaurant, and afterward, Mary gives Tim her phone number. Tim returns home to find a distraught Harry. It turns out that the same night as he met Mary, the opening night of Harry's new play had been ruined by one of the actors forgetting his lines at a crucial point. Tim goes back in time to put things right and the play is a triumph.

Having saved Harry's opening night, Tim tries to call Mary, but discovers that her number is no longer in his mobile phone. By going back in time to help Harry, Tim chose a path in which the evening with Mary never happened. However, he recalls that Mary was obsessed with Kate Moss. By attending a Kate Moss exhibition in town, he is able to run into Mary again. He strikes up an acquaintance with her but discovers she now has a boyfriend. Tim finds out when and where they met, turns up early and persuades her to leave the party before she can meet her future boyfriend. Their relationship develops and Tim moves in with Mary. He encounters Charlotte again by accident, and this time she tells him that she would be interested in pursuing a romantic liaison with him. Tim turns her down, realising that he is truly in love with Mary. He proposes marriage; she accepts and is welcomed into his family. Their first child, Posy, is born. Tim's sister, Kit, has not been so lucky and her unhappy relationship, failure to find a career, and drinking lead her to crash her car on the same day as Posy's first birthday.

Kit is seriously hurt but begins to make a good recovery. Tim decides to intervene in her life and does so by preventing her from meeting her boyfriend, Jimmy (Tom Hughes). When he returns to the present time, he finds Posy has never been born and that he has a son instead. His father explains that travelling back to change things before his children were born would mean those children would not be born. Thus, any events that occurred before Posy's birth cannot be changed, and Tim must accept the consequences as a normal person would. Tim accepts he cannot change Kit's life by changing her past but he and Mary help her to change her life in the present. She settles down with an old friend of Tim's and has a child of her own. Tim and Mary have another child, a baby boy.

Tim learns that his father has terminal cancer and that time travel cannot change it. His father has known for quite some time, but kept travelling back in time to effectively extend his life and spend more time with his family. He tells Tim to live each day twice in order to be truly happy: the first time, live it as normal, and the second time, live every day again almost exactly the same. The first time with all the tensions and worries that stop us noticing how sweet the world can be, but the second time noticing. Tim follows this advice and also travels back into the past to visit his father whenever he misses him.

Mary tells Tim she wants a third child. He is reluctant at first because he will not be able to visit his father after the baby is born but agrees. After visiting his father for the following nine months, Tim tells his father that he cannot visit any more. They travel back to when Tim was a small boy, reliving a fond memory of them playing on the beach, and after have a heartfelt, tearful goodbye. Mary gives birth to another baby girl, and Tim knows he can never see his father again. After reliving each day, Tim comes to realise that it is better to live each day once, and appreciate everything as if he is living it for the second time. The film ends with Tim leaving Mary in bed and getting his three children ready for school.

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