The Power Of Three is the name of Episode 4 in Series 7 (not including the classic series), the current Doctor Who season. Chris Chibnall (Life On Mars, Torchwood), who authored Episode 2, also wrote this one for the hugely popular BBC science-fiction television show.
The Power Of Three will feature the special military taskforce UNIT, whose purpose is to investigate and fight any major paranormal or extraterrestrial threats to planet Earth. The eleventh Doctor, Matt Smith, makes an extended visit with his companions Amy Pond (Karen Gillan) and Rory Williams (Arthur Darvill).
In this story of when the Doctor "came to stay", here are some of guest stars: Mark Williams ('Harry Potter' - 2002 to 2011) as Brian Williams (Rory's Dad), Jemma Redgrave (Bramwell - 1995 to 1998) as Kate Stewart, Steven Berkoff ('The Tourist') as Shakri. Also making appearances, as themselves, are physicist Brian Cox (Wonders Of The Universe) and business tycoon Alan Sugar.
Here are the "Next Time" and the full stand-alone official BBC trailers for Episode 4, The Power Of Three. It airs tonight - September 22, 2012.