No, it's the Tardis. Obvious when you think about it.
The episode has a sinister opening, with Uncle, Auntie and Nephew (an Ood) preparing Idris to have her soul sucked out and replaced with another one. Cut to The Doctor, Amy and Rory in the Tardis, bantering about past adventures. There's a knock on the door. They're in space. The Doctor opens the door and finds a message capsule from another Time Lord, "The Corsair". The message leads them to a junkyard world in a pocket universe, where the Matrix is sucked out of the Tardis.
It turns out to be a trap of course - while The Doctor searches for other survivors of the Time War, he sends Amy and Rory back to the Tardis and locks them in. He then discovers that House, the planet they are on, is a sentient planet that eats Tardises, and that The Corsair has been killed and his body parts used in House's patchwork servants. Meanwhile, House having heard from The Doctor that his Tardis is the last one left, has decided to hijack it and head for our universe in search of more food. But The Doctor has an ally - Idris, who has become a personification of the Tardis.
Neil Gaiman makes good use of Doctor Who's past in this story, pulling off the trick of using references to previous adventures in such a way that they add something if you know the reference, but don't take anything away if you don't. The Tardis comes across as pleasingly barmy, and the idea that she wanted to see the universe, so she stole a Time Lord and ran away is a neat symmetry. Best quote of the episode has to be the following exchange-
I just want to say, you know, you have never been very reliable.
And you have?
You didn't always take me where I wanted to go.
No, but I always took you where you needed to go.
Of course, we always knew that was how it worked, but now it's been said on screen.
But despite focussing on The Doctor's relationship with his Tardis, the episode didn't neglect Amy and Rory, and showed some real insights into their relationship too. Amy, who's normally full of mad bravado, was allowed to let the mask slip and be vulnerable. The quote "Hold my hand, Rory," when the Tardis took off under House's control was touching, and she was bought face to face with her greatest possible fear - losing Rory not only physically, but emotionally too. Then the real Rory turn up, and show us what she needs him to be - the reliable one, the one sane person in a mad universe. Rory had had a wonderful Genre Savvy moment earlier, when House asked, "Why shouldn't I just kill you now?" and he replied, "Because it wouldn't be any fun." Interesting that the Tardis thought that he was "the pretty one".
This week's cryptic bit - The river is the only water in your forest. River Song, perhaps? she did appear in Forest of the Dead, and also appeared in a forest in Flesh and Stone.
Note to Steven Moffat- ask Neil Gaiman back.