Now this is sumptuous. Dennis Spooner is practically giving a masterclass on how to write 'Doctor Who' here, creating a climax with rising tension in every aspect. Ian and Delos' escape sets up a dilemma for Barbara, Barbara's revelations to Tavius set up a dilemma for the Doctor and Vicki, and the Doctor's accident sets up the great historical event that, in retrospect, we've been leading up to all along. Again, when you look at this next to 'The Reign of Terror', you can see that Spooner has learned all the right lessons from his previous story. Instead of passive observers of history, the Doctor and his companions are now taking a causative role--it's a major shift in philosophy for the series, and in some ways the ultimate climax of the story is the scene between the Doctor and Vicki where she accuses him of causing the Great Fire of Rome. Hartnell's performance is brilliant here; it's clear that this is the first time he's ever shaken off his dispassionate, alien upbringing and thought of himself as an architect of history...and when his response is to giggle madly at the idea, you can understand that we've moved into a whole new era for the program.
Of course, the whole thing wouldn't work if everyone wasn't giving amazing efforts at acting. Derek Francis amps up his petulant, charming monster to entirely new levels as "Inferno" proceeds, his face alternately contorting in fury and crumpling in petulant sadness (and I'll digress for a moment here and giggle like a loon as Hartnell's knowing puns about his own impending death by vicious lion--that is some epic trolling there) before finally lighting with a gleam of madness as he sets Rome alight. Even Hartnell gets upstaged when Francis is on a roll, and that's a pretty impressive feat by this point in the show's history. He's absolutely magnetic everywhere else, to be sure; his mercurial, unpredictable performance allows the show to be whatever it needs to from episode to episode and moment to moment. It's not that he hasn't been this good before, mind you, but this is the story where he really gets to show off what he can do and demonstrates that he can act as an anchor when the program makes great leaps in genre and tone.
This is really one of the moments where the series takes a quantum leap in quality, realizing that it's capable of much more than it's been doing up until now and adding an entire new set of tools to its workbox. They've had jokes in previous stories, of course, but this is the point where they realize that Doctor Who can be a comedy as well as an adventure series without sacrificing tension. It works on a whole different level from the stories before it, and sets a new course for the entire future of the show. Oh, and also it's awesome.