Having saved Harry's opening night, Tim tries to call Mary, but discovers that her number is no longer in his cell phone. By going back in time to help Harry, Tim chose a path in which the evening with Mary never happened. He remembers her interest in Kate Moss and manages to find her at a Kate Moss exhibition. 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 with him before she can meet the boyfriend. Their relationship develops and Tim moves in with Mary. He encounters Charlotte again by accident and this time she makes it clear she would be ready to start a relationship. Tim turns her down, realising he really loves 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 herself. She settles down with an old friend of Tim's and has a child of her own. Tim and Mary have another child, a 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, experiencing each day's inherent unpredictability and stress, and the second time to appreciate its small joys and special moments without making any changes to the events that occurred (and avoiding changing any future events). 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. 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 a second time. The film ends with Tim leaving Mary in bed and getting his three children ready for school.
