2. The Mind Robber by Peter Ling
While most of the entries on this list have precursors in science fiction parallel universes, alternate timelines, even dream worlds The Mind Robber strays into pure fairy tale as the Doctor, Jamie and Zoe arrive in the Land of Fiction, a place entirely populated by fictional characters. This came after a very psychedelic first episode of materialising in white voids and the TARDIS breaking apart, but we won't go into that now. Encountering such characters as Gulliver and Rapunzel and having to reassemble Jamie's face so he stops being a cardboard cutout (again, let's not go there), the Doctor and his friends realise that something has drawn them here. And may not let them escape. Although they ostensibly do get out, one interpretation could be that the Doctor never left the Land of Fiction and is now himself a fictional character. Thereby rendering the adventures we have been watching him have ever since as not only fictional to us but also fictional to the Doctor as he is no longer in his own universe and is merely in a land of make-believe. I think that makes sense. Or maybe the meaning's just fictional.