Tim Lake grows up in Cornwall with his father James, mother Mary, uncle Desmond, and younger sister Katherine (“Kit Kat”). The morning after a less-than-great New Year’s Eve party, James tells Tim that the men of their family can travel back in time to moments they have lived before. Tim tests this by going back to the previous night’s party and changing a few events. When he returns, James discourages him from using his gift to acquire money or fame, and Tim decides to use it to improve his love life.

The following summer, Kit Kat’s friend Charlotte visits. Although instantly smitten, Tim waits until the last day to tell her; she tells him he should have told her earlier. Tim travels back in time to tell Charlotte in the middle of the holiday, but she suggests he wait until her last day. Heartbroken, he realises that she is uninterested in him and time travel cannot change anyone’s mind.

Tim moves to London to pursue a career as a lawyer, initially living with his father’s acquaintance Harry, a playwright. He visits a Dans le Noir restaurant, where he meets Mary, an American who works for a publisher. They flirt in the darkness, and she gives Tim her phone number. He returns home to a distraught Harry, whose play’s opening night has been ruined by an actor forgetting his lines. Tim goes back in time to help the actor so the play is a triumph.

However, when Tim tries to call Mary, he discovers that by going back to help Harry, the evening with her never occurred so he does not have her number. Recalling Mary’s obsession with Kate Moss, he attends a Kate Moss exhibition every day until he sees Mary.

Having never met Tim, she is confused but allows him to join her and her friend. During lunch, he discovers that she now has a boyfriend. Tim goes back to when they met, turning up before the potential boyfriend arrives, and persuades Mary to leave with him.

Their relationship develops, and Tim moves in with Mary. One night, he encounters Charlotte, who is now interested in him, but he turns down the invitation of intimacy as he is in love with Mary. Tim returns home and proposes. They marry, and shortly afterwards have a baby daughter, Posy.

Kit Kat’s relationship and employment struggles lead her to drunkenly crash her car on Posy’s first birthday. Tim decides to intervene: he prevents the crash and takes her back to avert a bad relationship. Returning to the present, he finds that Posy has never been born but he has a son instead. James explains that changing events prior to their children’s birth may alter the exact child conceived.

Tim accepts that he cannot solve his sister’s problems by changing her past; he lets the crash happen, ensuring Posy’s birth, and he and Mary help Kit Kat face her problems. She settles down with Tim’s friend Jay. Tim and Mary have another child, a boy.

Tim learns that James has terminal lung cancer and time travel cannot change it, as going back to remove his smoking would undo his and Kit Kat’s conceptions. His father has known his illness would come for some time, and so has been travelling back in time to extend his life and spend more time with his family.

He tells Tim to live each day twice to be truly happy: first, with all the everyday tensions and worries, but the second time noticing how sweet the world can be. Tim follows this advice; his father dies, but on the day of the funeral, Tim travels to the past to visit his father.

Mary tells Tim that she wants a third child. He is reluctant as he will not be able to visit his father again. Tim tells James, so together they travel back to relive a happy memory from Tim’s childhood, taking care not to change the experience to avoid causing changes to the present.

Mary gives birth to another girl. Jay and Kit Kat, very happy together, have their first child. The family accepts the loss of James, and Tim realises that it is better to live each day once only. He ultimately decides to not time travel at all and comes to appreciate life with his family as if he is living it for the second time.