dv said:

The next three Doctor Who stories form a kind of trilogy, even though there’s a change in actor for the eponymous character. I’ve seen them all multiple times.

The Keeper of Traken

Returning to N-Space through a Charged Vaccum Emboitment, the Doctor receives a call for aid from the Keeper of Traken. It emerges that Traken is being destabilised by hidden forces connected to the calcified Melkur in the garden.

The Doctor and Adric are aided by Consul Tremas (played by Anthony Ainley) and his daughter Nyssa. The interior of Melkur is suspiciously large and has round things on the walls…

The Master has run out regenerations and clings for bare life, and plans to use Traken’s Source to restore his power. This doesn’t work out, but he does manage to nab the body of Tremas and, restored to health, he escapes (now played by Ainley, as indeed he is for the rest of the Classic era).
This story features Geoffrey Beevers’s only on-screen performance as the decayed Master, who needs Traken’s Source because he’s run out of regenerations and clings to dear life. Beevers has since reprised the role in dozens of audio stories. He is the widower of Caroline John, who played Liz Shaw.

I suppose Tremas becoming Master would have been easily picked by anagram fans. Unlike the Doctor, the Master has a Tardis with a functioning chameleon circuit, and it takes the guise of a grandfather clock when he captures Tremas.

Next Time: We learn that the universe should have ended a long time ago, but has been saved by people chanting numbers.