In the TARDIS, the Doctor and Adric return from E-space and land in the Traken Union, a peaceful empire. The image of the elderly Keeper (Denis Carey) appears, asking for the Doctor’s help. The Keeper’s title is soon to pass on to Consul Tremas (Anthony Ainley), giving him access to the Source, which is the center of Traken’s technological advancement, but the Keeper senses some sort of evil. He believes that Melkur (voice of Geoffrey Beevers) is involved, an evil creature that arrived years ago, but became calcified in a grove like a statue. Tremas’s wife Kassia (Sheila Ruskin) takes care of the symbolic object. Soon, this responsibility will be passed to his daughter Nyssa (Sarah Sutton).
The Doctor and Adric land at the capital and visit the Keeper. Bodies begin to be found and, of course, they are blamed. The victims were killed by some sort of plasma weapon. Adric works with Nyssa to identify its signature, which reveals that it came from a TARDIS. Kassia visits Melkur, who speaks and moves and gives her a collar to wear, giving the alien control over her mind. She is able to get Tremas, the Doctor, Adric, and her daughter Nyssa arrested and has herself installed as the next Keeper in waiting. The Keeper dies, still warning of evil, and she takes the throne, but suddenly disappears, replaced by the statue of Melkur.
The Doctor and friends escape and cause a servo-shutdown of the Source to destabilize it and disconnect Melkur. As Adric and Nyssa prepare to activate it, the Doctor finds himself drawn into the statue, which is a TARDIS. He meets his old enemy, the horribly disfigured Master, who is on his last regeneration and wants to use the powerful Source to give him more. But when Adric and Nyssa initiate the servo-shutdown, it disconnects the Master and causes his TARDIS to malfunction. The Doctor escapes and Melkur disappears. A Consul takes the throne to restabilize the Source. After the Doctor and Adric depart, Tremas discovers a strange object in his rooms—a long-case clock—which is the Master’s TARDIS. He takes Tremas’ body and vanishes. Nyssa walks in, wondering where her father could be.
Tremas is an anagram of Master, but there is a line in Shakespeare’s Julius Caesar: “Kassia is as good a name as Tremas.” There is a Melkur in J.R.R. Tolkien’s Silmarillion. The name means “A fly caught by honey.” Apparently, when evil comes to Traken, it is calcified and passes harmlessly into the soil. Keepers live for a thousand years. The story was considered a triumph and much in it was praised by critics, including the gorgeous costumes, the art-nouveau sets, and the acting. The fact that Anthony Ainley’s Master bears such a resemblance to Roger Delgado’s Master is a happy coincidence, but a bit creepy. There are no earthlings at all in the story. Nyssa is not yet a companion, so this story is the only one in which the Doctor travelled with a male companion only; it would not happen again for 36 years.