Wikipedia
Logopolis
Logopolis is the seventh and final serial of the 18th season in the British science fiction television series Doctor Who, which was first broadcast in four weekly parts from 28 February to 21 March 1981. It was Tom Baker's last story as the Fourth Doctor and marks the first appearance of Peter Davison as the Fifth Doctor. This serial also marks the first appearance of Janet Fielding as new companion Tegan Jovanka, whilst Nyssa, played by Sarah Sutton and seen in previous serial The Keeper of Traken, returns and also joins the Doctor as a companion.