While it could, and indeed perhaps should, be argued that the entirety of Season One of Doctor Who represents a high water mark of both Sixties television in general & science fiction, The Edge Of Destruction should perhaps be held up as one of the prime arguments for that statement. By this point we still know comparatively little of the Doctor & his companions as they've shared just two TARDIS trips- one to the Stone Age after Ian & Barbara stumble upon the truth about Susan Foreman after meeting the curmudgeon she calls ' Grandfather' who pilots a police box known as the TARDIS through time & space. Next it was off to Skaro for a first meeting with The Daleks.
And to think Chesterton couldn't bring himself to believe that a thing that looked like a police box & had stood in the Totters Lane scrapyard could pull it off! But by the time we reach The Edge of Destruction it's the old girl who is the star of the show-
SUSAN: But why didn't the fault locator tell us?
DOCTOR: Well, the switch hadn't broken down, therefore the fault locator couldn't give us any recognition. You see, let me give you a demonstration. (using his torch) Now, look, when I put my thumb on there, the light comes on. And it only stays on so long as my thumb is pressing that switch. As soon as I take if off, a little spring inside releases the switch here and out goes the light.
SUSAN: Oh, I see. So if the spring were broken, it would be as if your finger were pressing it down all the time.
DOCTOR: Precisely. As simple as that. You know, my dear child, I think your old grandfather is going a tiny little bit around the bend. Well, I think you were very brave and I was proud of you.
The Type 40 was trying to warn everyone aboard that something was wrong, by means of a first real showing of its implied sentient nature! The fast return switch was stuck thanks to a dodgy spring, & the resulting temporal disturbances go some way to explaining everyone's incredibly odd behaviour- attempting to attack each other being among the sort of behaviour usually confined to small children in cars on long journeys, albeit not on so downright paranoid a level.
Initially the Doctor had accused Ian and Barbara of sabotage! Fearing that they had been taken over by some alien force. After the fault is discovered and the day is saved, Barbara is understandably upset by the Doctor's harsh words of accusation. And so, the Doctor is forced to do what he least enjoys - apologise, and admit that he was wrong about the two Coal Hill School teachers...
SUSAN: Grandfather? What about them? You said some terrible things to them. When I thought he was going to attack you, even I was against him.The journey back from the brink of temporal doom is notable in that only the four main travellers are present throughout & never leave their trusty transport, except at the very end of The Brink Of Disaster when they step out to find themselves in China. The year is 1289, which means they're about to pad into the first ''pure historical'' on The Roof Of The World, a jaunt on which they'll meet Marco Polo journeying to the heart of Kublai Khan's mighty Mongol empire. That story is notable indeed on a different level!
DOCTOR: Yes, I, I, er, well...
IAN: Don't bother to say anything, Doctor. You know there are times when I can read every thought on your face.
DOCTOR: Really? And I always thought that you were a young man without any recrimination in you. Well, as for you, young lady, well, you were absolutely right. It was your instinct and intuition against my logic, and you succeeded. I mean, the blackouts, and the still pictures and the clock. Well, you read a story into all these things and were determined to hold on to it. We all owe you our lives.
BARBARA: I, I.........
DOCTOR: You know, I really believe I have underestimated that young lady in the past, Chartow. Well now, we can all start again, eh? Yes, we can. Yes. But which? Hmm? What are you laughing at, dear boy? Oh, really, you are.....
BARBARA: Is your name Marco Polo?So now that the events of the two-part 'bottle episode' (written by story editor David Whitaker in just two days) had firmly united our band of time travellers as a team, they were ready to take part in Doctor Who's first history lesson, in the form of the series' first pure historical adventure.
POLO: It is, my lady, and may I ask who you are?
DOCTOR: Oh we're travellers yes. That's my grandchild, Susan, and that's Miss Wright, and that's Charlton.
IAN: Chesterton. Ian Chesterton.
POLO: My companions are the Lady Ping-Cho and the Warlord Tegana. We travel to Shang Tu.
BARBARA: Shang Tu? That's in China, isn't it?
POLO: China? I do not know this place. Shang Tu is in Cathay.
BARBARA: Oh, silly of me. Yes, of course, Cathay.
And so the foundations of early Doctor Who, at least up until The Highlanders are laid. Hurrah for 'Sexy'!