Here's the official description for the tenth episode of Doctor Who season four, "Midnight":
The Doctor is trapped, alone, powerless and terrified, on the planet Midnight... Soon, the knocking on the wall begins. Only a woman called Sky seems to know the truth – but as paranoia turns into a witch-hunt, Sky turns the Doctor's greatest strengths against him, and a sacrifice must be made...
Also, a few new bits about Steven Moffat's library two-parter which starts Saturday. One of the members of the expedition, Lux, is the grandchild of the guy who created the planet-wide library in the first place.
And there's a new and even dafter rumor about the end of the season. The Doctor dies and regenerates — but his severed hand regenerates also! And the severed hand turns into a complete person! Which means there are now permanently two Doctors. (Of course!) Also, David Tennant is rumored to be leaving after the 2009 one-off specials, plus a movie. [Doctor Who Forum]