10 MORE Unanswered Mysteries From Doctor Who
6. If River Has Met Earlier Doctors, Why Doesn't Ten Know Her?
When Professor River Song meets the Doctor in the Library, she knows him immediately. She even chides him for doing a very good job of pretending that he doesn't know who she is. He has only one answer to this, which is to say that he hasn't a clue who the woman is at all. Only then does River realise she's met him out of order again. However, there is a mystery in this.
At the first moment, she walks up to him with his Tenth face and knows him on-sight, without thinking it odd. This suggests that she has met this incarnation before. Further to this, the audio dramas have seen River meet Paul McGann's Eighth Doctor and Tom Baker's Fourth Doctor. So, why doesn't Ten remember her?
Even discounting these Big Finish productions, there is a fairly big mystery as to why he doesn't remember her. No moment of the show depicted his memory of her (nor her's of him) being wiped. What happened along the way? If, as the show suggests, she had just spent more than twenty years on Derillium with the Twelfth Doctor, shouldn't there have been at least a mention of it?