SPOILERS TO FOLLOW
England. 1938. Having barely survived a violent explosion on-board a spaceship, a violent expulsion from said spaceship and a rather sudden reentry into Earth's atmosphere without the benefit of any kind of spaceship, The Doctor is aided by a kindly British housewife named Madge Arwell. Helped out of the crater he created upon impact and given a lift back to the TARDIS, The Doctor promises Madge that he will find some way to repay her kindness.
Three years later, Madge is a war widow. Her pilot husband was reportedly lost over the English Channel several days before Christmas and Madge is determined to spare her children the painful news until after the holidays are over. Relocating to a relative's house, Madge is surprised to find the house being watched over by a mysterious "Caretaker", who has "improved" the home to make it more enjoyable for the children. Madge doesn't recognize The Caretaker as the "space man" she helped three years earlier and The Doctor doesn't see fit to educate her as to why he is going out of his way to give her family the greatest Christmas ever. But when The Doctor's final present goes awry, it will fall to Madge to save not only her own children... but an entire world.
THE GOOD PARTS
* Despite the title and the image of walking through a door into a magical forest, this story has nothing to do with Narnia on any level.
* Part of the magic of Doctor Who is the various wonders that we see as we travel across space and time. It is quite rare for The Doctor to be entirely clueless about the various beautiful things that he shares with his companions. But when it does happen - as it happens here with Matt Smith's delight at finding a forest of Christmas trees that grow their own ornaments - it is a wonderful thing.
* Madge is easily the best character yet to headline one of the one-shot Christmas specials. She is strong, capable of holding her own against a number of heavily armed security agents in an alien world and not going to pieces even as she struggles to save her children... and more.
* The final scene, in which The Doctor proves himself to be more human than he ever thought, is picture perfect. Crying when you're happy. How human.
* The episode's greatest strength is also its' greatest weakness. Because as awesome as Madge is as she solves all the problems in the episode... when you get right down to it, all of the problems in this episode were caused by The Doctor being an idiot. He failed to consider that children always try to sneak a peak at their presents early. He didn't fully explore the forest or the time period before creating a time portal. And he doesn't really do anything to save the day. As The Doctor himself notes, it was all down to Madge. And a bit of dumb luck.
* This is a minor point, but I can't help but wonder what happened to the other two soldiers on Madge's husband's plane. Yes, the focus is upon how HE was able to make it home for Christmas and how his wife inadvertently became the guiding beacon he needed to make it home safe... but it still would have been nice to see them come out of the bomber and get an invitation to join the family for Christmas dinner.
THE FINAL VERDICT
Set on a smaller stage than the other recent Christmas specials, this episode is a more traditional Christmas story than we usually get. It's basically all about the power of a mother's love for her family and - ironically - it is the outsider Doctor who causes all of the story's complications. The implications that sometimes The Doctor would be better off not getting involved seem to be a likely foreshadowing of what is to come in Series 7.