The Doctor's Wife

Thought you were safe in the TARDIS? Think again. The madman in the box has been locked out, and even the matrix has been removed, leaving Rory and Amy trapped in the House-controlled TARDIS. Corridors with threatening messages scrawled in blood, time-warped seperations, and disappearing lights would be terrifying enough in any circumstances, but this is in the TARDIS, which turns ordinary nightmare fuel into the H-bomb of terror.
Even worse, this is not a Steven Moffat episode, but Neil Gaiman's episode "The Doctor's Wife," in which a little white box lures the Doctor to the junkyard in the bubble universe, on a senient astroid occupied by the green-eyed Ood Nephew, the patchwork couple Uncle and Aunt, and the mentally unhindged Indress, who babbles about kissing, thieves, and a big, sad, word.
While past episodes have hinted at the TARDIS's senience, "The Doctor's Wife" brings those suggestions to a gloriously satisfying conclusion for fans, not to mention providing rich fanfic fuel by creating a living embodiment of the TARDIS.
The TARDIS and the Doctor--the Madman with a Box--two nearly inspirable icons of the show. In fact, the chamelion-circuit challenged police box became so well-known that the BBC could take the rights away from the police. And now, almost fifty years after we first met her sitting in a Trotter's Lane junkyard, we get to hear the TARDIS's perspective.
Even with the obligatory snarky remarks, verb tense difficulties, hyperactivity, and biting, the actress beautifully shows the view from a new perspective. Lines such as “I took you where you needed to be,” show the brilliant inanity is mutual. But of course, it can’t last. Putting an 11-dimensional interface into a human body is only slightly less harmful than holding the Time Vortex in a human head or being the Time Lord mind/human body half of a biological metacrisis. In one way, the end is most similar to Doomsday, because the two minds are still alive, but separated by unyielding walls. But I found it even sadder, because Idris/TARDIS is still with the Doctor, watching everything he does, listening to him, but never able to answer. This was “the time [they] talked,” and despite the pain, it was worth every moment.
The saddest part was this conversation near the end. Idris had mentioned this several times, but I didn’t suspect what it could be.
Idris: I've been looking for a word. A big, complicated word but so sad. I've found it now.
The Doctor: What word?
Idris: Alive. Alive. I'm alive.
The Doctor: Alive isn't sad.
Idris: It's sad when it's over.