Doctor Who: State of Decay (Part 4)

"State of Decay (Part 4)" is the fourth and final part of the "State of Decay" storyline, which ran through episodes 13-16 in series 18 of Doctor Who. The episodes originally aired on BBC1 from November 22nd to December 13th, 1980. It was directed by Michael Hayes and written by David Fisher, Douglas Adams and Graham Williams, all credited under the single pseudonym of David Agnew. In this episode, Scaroth captures Romana and blackmails her into completing Kerensky's work on his time machine. The Doctor fails to prevent him from activating it, and must follow him back to Earth's primordial past to stop him from preventing the development of the human race.