Spoiler Level: High
Wow. One expects big things when you hear the name "Neil Gaiman," but this was just magical.
Since Steven Moffat said we were going to learn a lot about River Song this season, when the title "The Doctor's Wife" was announced I figured that meant this episode would be focusing on her. Then we saw the previews and River was nowhere to be found and I thought, "Oh, no, not another fake-out like Jenny Who." But Neil Gaiman delivered the goods big time. Who's truly the Doctor's wife? Well, who else but the TARDIS?
Having the soul of the TARDIS being deposited into a human body so we can hear the TARDIS's point of view is just pure genius. I loved that she started out immediately calling the Doctor "Thief!" since he stole her in the first place, and then the twist that she looks at it as she was stealing a Time Lord so she could see the universe was just absolutely wonderful. They were both destined to go on this trip together.
So, time for me to get nerdy on the age issue again! Idris (and I love that name-- it's like an abbreviation of "I, TARDIS." Anyway, I/DRIS) states that she and the Doctor have been traveling together for 700 years. So the Doctor must have stolen her and fled Gallifrey when he was around 200 years old, which makes sense. I would have loved a Susan reference in there as well, but I'll have to settle for the flood of other delicious references, such as the previous control rooms, the way the Doctor has always referred to her as "old girl" in the past and "sexy" more recently, and their argument on why she rarely takes him where he actually wants to go.
And corridors!! We get to see some TARDIS corridors! I was very pleased with that. I have mixed feelings about the secondary control room being the previous one. On the one hand, I was genuinely surprised, since I had thought that they'd literally destroyed that set, so that was really cool. On the other hand, it's a bit soon to evoke much nostalgia for me, so I was hoping to see one of the earlier classic control rooms. No matter-- this way still worked great.
This is my favorite episode of the season so far, and it may end up being the best episode of this season, period.