The Doctor is not who he was.
Barbara notices that the "faint trembling" while it's in flight has stopped. But the Doctor is asleep in an armchair, the first time he hasn't supervised the landing. When they wake him, it's not the usual take-charge, imperious old man they know:
The DOCTOR is asleep in his chair)He's disoriented, here, but Hartnell plays him as also gentler than usual. It's an interesting sequence, comic and a little disturbing.
BARBARA: And you know how the ship has a faint sort of trembling while we're moving? Well, I suddenly realised that it had stopped.
IAN: I believe you're right, Barbara. I think we have landed. But the Doctor's never slept through a landing before.
BARBARA: Doctor. Doctor!
BARBARA: Wake up.
DOCTOR: Hmm? What's the matter? What is it? What is it? Oh good gracious me! Don't tell me I went off to sleep.
IAN: Yes, you did.
IAN: At a very critical time. Oh well, I suppose it did you a world of good.
DOCTOR: Deep in the arms of Morpheus, hey my boy? Well, I feel a bit sticky. I must go and have a wash.
BARBARA: Oh, but Doctor, the trembling's stopped.
DOCTOR: Oh, my dear, I'm so glad you're feeling better.
BARBARA: No, not me, the ship.
DOCTOR: Oh, the. Oh my dear, I'm so sorry.
IAN: Doctor, we appear to have landed while you were asleep.
DOCTOR: What? Oh, I say, I must never allow this sort of thing again now, must we? No. Well, all we have to do is to turn the power off.
BARBARA: Then we have landed.
DOCTOR: Yes, er, excuse me, materialised, I think, is a better word.
Which is a good way to describe this brief, two part story.
The whole reality and illusion genre has its detractors, but this is Doctor Who's first trip to that well--and it's still only January, 1965. The Stunt Man is still 15 years in the future, to say nothing of overtly sci-fi-fi variations of the genre like The Matrix (a hair under a quarter of a century off) or Inception (35 years out).
Because we don't open with the Doctor and the TARDIS as I have. No, we open on the remnants of a badly damaged spaceship, where the two remaining crew members live in fear of the one member they have met of the dominant species of the planet Dido (named after a widowed queen who, deserted by her lover, kills herself), the choleric Koquillion. Koquillion, a spiky creature, threatens and reassures over and over again, insisting to the orphaned Vickie and her crewman Bennett, who has lost the use of his legs, that the other members of his species want them both dead.
But the Doctor recalls Dido as a peaceful planet.
Barbara meet Koquillion, who kills her. The Doctor and Ian then find themselves hunted by a fearsome beast. The beast attacks Vickie. The Doctor speaks to Bennett.
None of these statements is true, and yet that's what we see on screen.
Everything you know is wrong.
Barbara is alive (hidden from sight by Vickie), the "fearsome beast" is Vickie's pet, a vegetarian, friendly animal native to the planet. Barbara kills it, thinking she is saving Vickie, only to break the younger woman's heart.
Koquillion is Bennett (who can walk fine) who is wearing the ceremonial dress of the Didonians, whom he killed when they welcomed the crew to the planet (Bennett, a murderer facing charges, killed the rest of them, too.) We hear the Doctor answered by Bennett, but he's not there, his voice has been recorded.
One last thing happens that subverts what we think we know (although a brief moment in Marco Polo might have tipped us off); the frail elderly Doctor lies in wait for "Koquillion" and, in a manner prefiguring Tom Baker at his most severe, accuses him of all his misdeeds. When Bennett physically attacks him, and the Doctor fights back spiritedly--he even disarms Bennett (though he loses his own sword), holding him back long enough that two of the remaining Didonians can charge Bennett--who, faced with younger opponents, staggers back in fear, and tumbles to his death.
Among all these deceptions, there are some moments of truth: Barbara's defensiveness and then her shame at having killed "Sandy"(the allegedly fearsome beat); Barbara's and Vickie's mutual apologies; the Doctor's paralyzed response to asking Susan--no longer there--to open the TARDIS door.
And the Doctor's immediate rapport with Vickie.
This could have been awful; it's all on Hartnell and Maureen O'Brien. He is far gentler and kinder than we've seen him before, he's like a man coaxing a cat to come to him. O'Brien, as Vickie, plays the needy young woman who has been tormented by Koquillian for too long, and who needs hope and reassurance so desperately. The two bond, and he invited her into the TARDIS, a conclusion Barbara and Ian have already reached themselves.
So off they go, our travelers, having found some truth in friendship, only to land on an unstable surface, and topple into the next story.