An interesting enough story from Christopher Bidmead, but the real draw and drama to this one is the buildup to the departure of Tom Baker. You can kind of feel the immanent funeral march throughout. The cloister bell (its first appearance), the Watcher, the foreboding music, the themes of entropy...all give an uneasy feeling of dread.
You can also sense how they're setting up the next Doctor's era, too. Tegan, played by Janet Fielding, is introduced in episode one, and Nyssa is brought back in the last moments of episode two...setting up the companion transition team with the continuing Adric.
Anthony Ainley is now firmly The Master, after taking Tremas' body in the final moments of The Keeper of Traken and he wastes no time in becoming the Time Lord we love to hate.
Really good stuff, and a sweet, poignant regeneration scene, too. The end of a very special era in DW, and the beginning of another...