Doctor Who: Arc of Infinity (Part 4)

"Arc of Infinity (Part 4)" is the fourth and final part of the "Arc of Infinity" storyline, which ran through episodes 1-4 in series 20 of Doctor Who. The episodes originally aired on BBC1 from January 3rd to January 12th, 1983. 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.