It'll be all over the internets by this morning, so anyone wishing to avoid spoilers for the penultimate episode of Series 8 (which may or may not be a lead-in to the finale) should unplug their computers and avoid all geeky media outlets between now and November. Spoilers after the break... You have been warned.
Still here? Okay, first lets get this out of the way:
A tantalizing glimpse of a script page reveals this episode's working title is Death In Heaven, written by Stephen Moffat, and features the return of UNIT as led by Kate Letherbridge-Stewart and her daughter Osgood (who this time is sporting a wardrobe inspired by the Second Doctor) squaring off against the new Nightmare in Silver style Cybermen and the "Gatekeeper of the Nethersphere," aka "Missy," played by Scottish actress Michelle Gomez.
The scene, set outside St. Pauls, involves The Doctor talking to the "Missy" character, who has possibly just been arrested by UNIT. According to "Missy," the dead are "coming back all shiny and new," and "within 24 hours the human race will cease to exist."
Meanwhile, Kate Stewart is pacing back and forth, talking to someone on her cell phone saying "Alien incursion confirmed, [something] 12 - 12 -12" and to "Tell Geneva the [something or other] protocols are active" and warn the president than an invasion is imminent (or something to that effect.)
There then appears to be some sort of visual effect happening (cued by the director shouting "bang" several times), likely loads of Cybermen teleporting in, and the armored UNIT troopers, plus a number of plain-clothed individuals all draw guns and look about. The Doctor shouts something, and the soldiers and civilians with guns look up and point their guns skywards. There are flashes of light, and the Doctor, Kate and Osgood start looking around where the light is.
The Missy character is held by UNIT guards and The Doctor begins speaking to her, saying "tell me more..." She shakes off the guards, and they raise their guns threateningly. She moves towards The Doctor and speaks to him, touching his lapels and his jacket. One of the uniformed UNIT guards breaks out some sort of new-fangled stun dart gun, shooting her in the neck, and she collapses to her knees, saying "Ooh, that's new."
The Doctor is understandably peeved about this, shouting "No, no! I need her awake!" He begins talking to Kate Stewart, and says something like "There are Cybermen all over the world." Kate says something back to him, and he replies "Shush now, I'm trying to count!" quite aggressively, whilst looking up into he sky. Osgood says "Eighty seven" and he looks at her.
He then says something about St. Paul's (possibly a reference to The Invasion) while Kate Stewart is back on her phone saying "Protocol something-or-other." The Doctor is about to rush off, and one of the soldiers walks up behind him and stuns him in the same way as Missy. As he's collapsing, he says "Stupid, stupid!" calls out for Clara, and then whispers something to Osgood before he passes out.
EDIT: Here's some video from shooting, which seems to be more or less the scene described above.
Also, at some point, Kate Stewart confronts the Cybermen, possibly claiming to be the representative of Earth and/or UNIT, and throws down a damaged 'Invasion' style Cyberman head as proof she means business. Now that's a UNIT leader with balls!
Oh yeah, and this photo of some sound recording paperwork lists both episode 11 and 12 together as being directed by Rachel Talalay. It may just be the production block, as they have been known to double bank shooting for episodes at the same time, but it might also be an indication that what we're looking at is the second half of a two-parter.