Episode 115 of Doctor Who, Keeper of Trakken, begins a rather long mytharc that will see Nyssa and join the Doctor and the ever annoying Adric. This episode, along with Logopolis and Castrovalva is also available in a three Doctor Who DVD set New Beginnings.
The always anoying Adric and the Doctor are brought to Trakken by its Keeper. Something is wrong on the most harmonious planet in the universe and the old Keeper wants the Doctor’s help in solving the mystery before it is too late. A statue name Melkur that has been taken care of for a long time by Kassia, who is now married to one of the planet’s consuls / rulers, is not as harmless as it seems to be and Kassia is under its spell.
The evil force behind Melkur is disrupting Trakken’s thousand years harmony and wants to take over Trakken. Will the Doctor solve the mystery in time? He might with the assistance of Nyssa, Kassia’s stepdaughter.
Anthony Ainley, who plays consul Tremas, would soon take on the role of the Doctor’s nemesis The Master.
You could read a lot of symbolism in this story about people whose bond and a benevolent ruler controlling some kind of unifying force being vanquished by someone honoring a statue / god. Of course, this is just TV.
To be honest, I am a new school Doctor Who David Tennant fan but that is not the reason why I find Tom Baker a rather uninteresting Doctor: he seems to bring very little personality to the role. Baker does get a few good lines here and episode 115 of the Doctor Who series Keeper of Trakken is above par both in terms of writing and setting. The evil Melkur is just another guy in a rubber suit but what do you expect?