Doctor Who: The Keeper of Traken (Part 4)

"The Keeper of Traken (Part 4)" is the fourth and final part of the "The Keeper of Traken" storyline, which ran through episodes 21-24 in series 18 of Doctor Who. The episodes originally aired on BBC1 from January 31st to February 21st, 1981. 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.