A really exciting Tom Baker story. The Melkur poses an intimidating threat over proceedings, and the reveal that it's the Master's TARDIS makes sense of a lot of the events that transpire.

I love the concept of the Keeper, as the one who inhabits the planet's source and maintains the planet's peaceful atmosphere. It's a bold and clever concept, that gives this fantastical feel to Traken society.

Nyssa's debut is overall a stronger companion introduction than Adric's. She moves the plot forward more than Adric does in his first story, and the only downside is that we don't get to see her reaction to the Master merging with Tremas. I think it would be nice had Nyssa arrived just as the Master and Tremas had merged, rather than after the process had already completed.