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A number of episodes have established that time can be rewritten and history changed without dire consequence, however there are "fixed points in time" that cannot ever be altered because to do so will fracture time. Many fixed points are major events with impact on history, such as World War II. Other events are smaller and relate to individual people's deaths. When Rose attempted to stop her father from being killed in "Father's Day", it nearly caused the end of reality. More recently, the death of Clara Oswald in "Face the Raven" was described as a fixed moment in time, and even though the Doctor used Time Lord technology to extract her a moment before her death ("Hell Bent"), and she subsequently began her own independent travels, she must still someday return to the location of her death and allow her life to end in order for the universe to continue. According to the Doctor, Time Lords can sense when they can or cannot alter history ("Dark Water"), and it has been implied that once the Doctor (or anyone) learns of the exact date of a person's death, that moment becomes fixed and in some cases the Doctor cannot even attempt contact with that person ("The Angels Take Manhattan").