Matt Smith and David Tennant star in the 50th anniversary special, as a mystery at the national gallery in the present day and an invasion of shape-shifting aliens in Elizabethan times bring two incarnations of the time lord together. A greater crisis looms with the arrival of a mysterious third Doctor, who is facing a battle for the fate of the universe and the most terrible decision of his many lives. With Jenna Coleman, and guest starring John Hurt, Billie Piper and Joanna Page.