Eleven’s Conundrum

An eleven-year-old girl appears in the TARDIS with no teleporter or any transport module, calling herself a 'reflection of a drifting past'. Keirdwen believes she has come from an alternate universe and clearly does not belong in a child's body. She is constantly being questioned by a very suspicious Doctor, who cannot accept her existence. Convincing the Time Lord to search for Keir's supposed planet of origin and getting her there are two entirely different things. Where does she, a lost girl from no time at all, really belong? And why has she found herself in the only sentient time traveling machine in the universe?