An old woman lived on a planet by herself. She was a telepath, and couldn't stand all the voices in her mind. She sealed herself off from the rest of the universe. Because she was lonely, she invented a companion for herself, a cat-like creature that the Second Doctor named Marmaduke. Marmaduke amplified her telepathic powers.
Because of her enhanced telepathy, she could once again hear voices in her head. She put the idea of taking Marmaduke away into the Doctor's head. When the Doctor tried to take Marmaduke back to her, she masked herself so he wouldn't be able to find her. However, Marmaduke's absence caused her powers to weaken, so the Doctor brought Marmaduke back a few centuries after he had taken the "cat", so that the old woman was no longer able to hide her presence. (PROSE: Constant Companion)