"Dr. Who" travels in time to meet the young Viking girl he saved who is now all grownup and is a highway robber of some sort.
In the sixth episode of "Dr. Who" season nine, the Doctor travels to 1651 to find a woman robbing somebody in a highway. This woman is looking for an amulet which also happens to be the Doctor's main objective in his jaunt through time. According to Entertainment Weekly, as it turns out this woman is no stranger to him as she was the Viking girl named Ashildr he once saved. However, the woman denies any knowledge of what the Doctor is talking about and says she has no recollection of the name she is being called by the stranger in front of her.
She tells the Doctor that she is now called "Me" and she does not belong to anyone either as somebody's mother, daughter, or wife. She insists she is her own companion. It was also revealed that she is already 800 years old and that her journal contains stories that even the wildest imagination will have trouble concocting. She was once a queen who ended the Hundred Years' War, rescued an entire village from falling prey to scarlet fever. She tells the Doctor why she chooses to forget everything that has happened to her in such a long span of time - it helps her avoid the pain.
In a post by The Telegraph, "The Woman Who Lived" discussed something about immortality that many shows that feature it never seem to explain - how the brain of a human being is able to process hundreds of years' worth of memories. Ashildr is a prime example when she said she has trouble remembering everything that has ever happened to her which is why she kept a journal. But even then, her own recordings can sound alien as there only small bits remaining in her mind.
