The BBC have released an official synopsis for episode four of Series 9 and part two of two, Before the Flood, which you can read below:
On a remote Army outpost, a fearsome alien warlord – the Fisher King – sets in motion a twisted plan to ensure his own survival. The ripples will be felt around the universe. Is this chain of events inevitable? And can the Doctor do the unthinkable?
Intriguingly, the BBC’s listing also confirms that Peter Serafinowicz (Star Wars: Episode I – The Phantom Menace, Guardians of the Galaxy) will be starring in the episode – rumours indicate that he will voice the Fisher King, the episode’s villain.
Before the Flood airs on Saturday 10th October on BBC One.