31 January 1981
Anthony Ainley (Tremas*), Sheila Ruskin (Kassia), Sarah Sutton (Nyssa), Denis Carey (The Keeper) [1-2], John Woodnutt (Seron) [1-2], Margot van der Burgh (Katura), Robin Soans (Luvic), Roland Oliver (Neman), Geoffrey Beevers (Melkur)*, Liam Prendergast , Philip Bloomfield  (Fosters)
|Written by||Johnny Byrne|
|Directed by||John Black|
|Produced by||John Nathan Turner|
The Union of Traken is governed by a Keeper gifted with the powers of the Source. The current Keeper is nearing the end of his thousand-year tenure, however, and asks The Doctor and Adric — who have escaped from E-Space — to go to Traken and stop an evil he believes is plotting to destroy the Unedition.
But the source of the evil, the Melkur, has already infiltrated the Consuls of Traken, and has The Doctor declared a criminal. Allying himself with Consul Tremas and his daughter, Nyssa, the Time Lord must uncover the true power behind the Melkur — someone who knows The Doctor of old.