The major twist in "About Time" is that it evolves from a romantic comedy into a profound story about family, loss, and the appreciation of life, with the relationship between Tim and his father becoming the central focus. A key plot point reveals that Tim's father has terminal cancer, and has been using time travel to extend his life and spend more time with his family. Time travel cannot cure him.
A critical rule of time travel is revealed when Tim tries to alter his sister Kit Kat's past to prevent a bad relationship and a subsequent car crash. When he returns to the present, he discovers his daughter, Posy, has been replaced by a son. His father explains that if he travels back to a time before his children were conceived and changes anything, the specific child born may be different. This rule creates the film's central emotional dilemma: after his father's death, Mary expresses a desire for a third child. Tim realizes that once the new baby is conceived, he will no longer be able to travel back to see his deceased father, as doing so would predate the third child's conception and risk changing them.
In a deeply poignant climax, Tim decides to have the third child and visits his father one last time. They travel back together to a cherished memory from Tim's childhood for a final goodbye. Ultimately, Tim learns his father's secret to happiness—reliving each day twice—but takes it a step further. He stops time traveling altogether, choosing to live each day once, but with the full appreciation as if it were his second time through. The ending reinforces the film's message that the true gift is not the ability to change the past, but the ability to live fully in the present.