WARNING! As the title character would say, Spoilers:
We got it; River Song meets the Twelfth Doctor. This is a wonderful comedy of errors. River believes there are only twelve possible faces of the Doctor. Time Lords have 12 regenerations therefore 13 incarnations. The Tenth Doctor used one to keep being him. River did not know that the Doctor gained 13 more incarnations, hence the comedy ensuing.
The Doctor gets to do the whole the TARDIS is bigger on the inside routine. He thought all the previous reactions were never up to snuff. River is looking for a surgeon, and her henchmen confuse The Doctor with a doctor. The comedy is some of the best we have seen in a long time. We even get the Doctor calling River sweetie and warning her of spoilers. Yes, as the episode title implies we meet some of River Song’s other husbands. I won’t elaborate; I don’t want to give too many spoilers.
This adventure occurs just before our first introduction to River Song, which if folks remember led to her untimely demise in Series 4 episodes 8 & 9: Silence in the Library & Forest of the Dead. It does something Doctor Who does well, bittersweet. The episode hints this may be the last we see of River Song. I can think of a few ways around that possibility. That is just me.
This was everything we could get in both a River Song episode and a Christmas special. They played with the roles; the Doctor got to be a companion of sorts. They switched the dynamic between river and the Doctor. This reversal allowed the Doctor to see how he impacted those that he loves, how they are when he is not around. Overall I feel the best episode of the series. I give it an A+.