Episode One of Tom Baker's final story starts with an unprecedented feeling of doom. It's not just the death of a poor policeman in the opening scene (policemen have never fared well in Doctor Who). It could partly be because of Paddy Kingsland's melancholic score, too. That first scene struck me as a child watching this for the first time, not being even remotely aware that I would be witnessing the end of the only Doctor I ever knew at the time.
Tom Baker's performance is also gives this episode that down feeling. He's never looked more tired than in this episode, wandering around the TARDIS, complaining about the entropy that's engulfed the TARDIS, but really complaining about the effect the advancing years have had on his Doctor. His line "Sometimes I think I need to run a tighter ship" could just as easily have been written by John Nathan-Turner, who has been hoping and planning for a story like this since he took over as producer at the beginning of Season 18. Not that he disagreed intensely with Tom Baker as The Doctor. It's just that Baker wasn't his Doctor.
What really gives this episode an "end of the world" feeling, though, is that certain events happen that have never happened to The Doctor before (or if they have, the circumstances haven't been this drastic). TARDISes materializing inside other TARDISes is something that's happened before in The Time Monster, but because of The Doctor's reaction to it here, it seems much more dangerous. But it's the first appearance of who we'll find out to be the Watcher that, as a 12-year-old, made me scared for the first time ever in my young life as a Doctor Who fan. The Doctor didn't know who this strange, white being was, standing across the road, and when The Doctor didn't know, I didn't know. And what's more scary to a 12-year-old than the unknown?
A brilliant, gloomy start to the end (which, of course, has been prepared for).