The BBC have released the official synopsis for Before the Flood, the fourth episode of Series 9.
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?
It airs Saturday 10 October on BBC One, at the late time of 8.25pm.