Hartnell’s in fine form in this episode. He’s gloriously tetchy when railing against the Sensorites (who refuse to give him the lock of the TARDIS back, which he wants so he can access his equipment to find a cure for Ian). They offer him their own laboratory facilities, which he accepts with very ill grace, but he can’t help himself and raises his voice on several occasions – even though he knows it causes the Sensorites pain.

Therefore it falls to Susan to act as the peacemaker between the Doctor and the Sensorites. It’s a nice, albeit brief, character moment for Carole Ann Ford who is clearly attempting to make the most of the thin material she has (after a strong start to the story, Susan is fading into the background again).

The Doctor’s in his element as he attempts to find a cure (surrounded by test-tubes you feel he’s very much in his natural environment). But the ease at which he does so is another weakness of the story. With six episodes to fill you’d have assumed they could have stretched it out a little longer and even when the City Administrator intercepts the antidote it doesn’t really matter, since Susan simply obtains another dose. Doctor Who and the Silurians has a better example of this type of plot-thread – after the Doctor is kidnapped by the Silurians his formula to stop the plague goes with him, so Liz has to frantically attempt to reassemble it from his notes.

The Doctor’s very pro-active in this episode and therefore quite different from the more self-centered character we saw earlier in the season. He travels down to the aqueduct in order to examine the source of the poison and, leaving his Sensorite guide at the entrance, ventures inside. When the First Elder and the others learn of this, there is general consternation.

1ST ELDER: Other expeditions have tried and failed. Most of our men do not return, and those that do speak of terrible things.
IAN: Well, I’ll have to go myself.
SUSAN: No you won’t.
IAN: We can’t stay here, Susan.
SUSAN: You’re too ill, Ian.
IAN: I’m not that ill.
SUSAN: All right. We’ll need someone to show us the way.
1ST ELDER: I beg you to change your minds. You cannot save your friend.
IAN: We’ll never know till we try, will we?
1ST ELDER: These people have fine qualities. The Second Elder and I have misjudged them, and I will tell him so.

The closing moment of the episode, as the Doctor hears the roar of a mysterious creature is an iconic one and also serves as a good example of Hartnell’s acting abilities. Later Doctors might have used the opportunity of an end-of-episode closeup as a chance to play to the camera, but Hartnell is very restrained. He keeps his face immobile whilst his eyes dash from right to left. It’s a very good indication that less is more and a small example of Hartnell’s class.