I mention this, not to be harsh to the actors under those masks, but because Hartnell gets a lot of stick for his flubs, and the fact is, they abound around him.
So, The Sensorites is, in essence, a story of recognition, of becoming undeceived. The Second Elder recognizes that, absent their insignia of office, Sensorites are functionally identical to all but their closest family-group members--and uses this knowledge to kill his way toward the top, in the finest House of Cards way (he lacks the panache of Ian Richardson, so he, in another, WestWing steal, he gets Underwood). As he begins with motives that are fear-driven, but not selfish, he also discovers his own lust for power and cruelty.
The First Elder realizes that he has been naive (cue Anthony Ainley!) in assuming his people incapable of greed, malice, deception. The most likable of the Sensorites, and the most complex, he also comes to recognize that humans cannot all be lumped together--that they can be good, brave and kind (like the TARDIS team) as well as grasping, greedy, etc.
Susan recognizes that, although she is not unhappy traveling, she wants "to belong somewhere." To go home, even if it's not her literal home. In a conversation that touches on Susan's views on Barbara and humanity, we get the first description of the Doctor's yet-unnamed world:
SUSAN: Thank you. Please find them, Barbara.The Doctor--he realizes that Susan is growing up, growing away from him, showing gifts that he does not expect, and cannot control. He pleads with her to maintain the status quo, for a little longer:
1ST ELDER: A very capable human being.
SUSAN: Yes, she is.
1ST ELDER: Gentle, yet with strong determination and courage.
BOTH: I was going to
SUSAN: I was going to say, why do you trust your people?
1ST ELDER: Why do you want to make me doubt them?
SUSAN: Trust can't be taken for granted. It must be earned. I trust you, but only because I know you.
1ST ELDER: But Susan, our whole life is based on trust.
SUSAN: Yes, and that might be your downfall. Look you don't trust the ground you walk on until you know it's firm, do you. So why trust your people blindly?
1ST ELDER: When I listen to you, you who are so young among your own kind, I realise that we Sensorites have a lot to learn from the people of Earth.
SUSAN: Grandfather and I don't come from Earth. Oh, it's ages since we've seen our planet. It's quite like Earth, but at night the sky is a burned orange, and the leaves on the trees are bright silver.
1ST ELDER: My mind tells me that you wish to see your home again, and yet there is a part of you which calls for adventure. A wanderlust.
SUSAN: Yes. Well, we'll all go home some day. That's if you'll let us.
1ST ELDER: I think I will.
DOCTOR: What's the matter, my child?As for Ian and Barbara, they discover that, improved though the Doctor is, he is still not entirely trustworthy, as the ending demonstrates:
SUSAN: I had a talk with the senior Scientist just before we left. It seems that the Sensphere has an extraordinary number of ultra high frequencies, so I won't be able to go on using thought transference.
DOCTOR: Oh, I don't know. It's rather a relief, I think. After all, no one likes an eavesdropper about, do they. No, I think you obviously have a gift in that direction. When we get home to our own place, I think we should try and perfect it.
SUSAN: When will we get back, Grandfather?
DOCTOR: I don't know, my dear. This old ship of mine seems to be an aimless thing. However, we don't worry about it, do we? Do you?
SUSAN: Sometimes I feel I'd like to belong somewhere, not just be a wanderer. Still, I'm not unhappy.
DOCTOR: Good, good.
IAN: Well, here we are.Oops.
DOCTOR: Always last. I very nearly went off without you.
BARBARA: We were saying goodbye to John and Carol.
DOCTOR: Let's have a look at Maitland and see him off, shall we?
(The scanner shows the ship heading into the stars)
IAN: Well, at least they know where they're going.
DOCTOR: Implying I don't?
IAN: I didn't mean anything
DOCTOR: So, you think I'm an incompetent old fool, do you?
IAN: Now, Doctor, I never said that.
DOCTOR: Since you are so dissatisfied, my boy, you can get off the ship. At the very next place we stop, I shall take you off myself, and that is quite final. Carry on.