Once again, it's amazing how bloody watchable the main cast makes this series. It is really hard to impress upon people who've grown up thinking of Doctor Who as a show with a fundamentally changeable dynamic, where cast members rotate regularly on a season-by-season basis, just how much thought and effort went into making every one of the series leads an important character in their own right and just how much this pays off on a week-by-week basis. In this episode, there's no less than three scenes that could have been mundane time-stretchers (and in the Aaru film, they were exactly that) transformed brilliantly by the chemistry among the four leads.
The first is when Ian is stuck in the Dalek machine. Again, from a plotting standpoint this is nothing more than a stall; it plays for time during the escape from the Dalek city that has to take up the better part of a whole episode, it provides a cheap "race against time" thrill as the Daleks try to cut the door down before Ian can get out and get away in the lift, and it gives a brief shocking moment when the sequence is edited misleadingly so that it looks like he failed.
But as played, it's something much better. William Russell plays Ian's demand that the others leave him with an utterly believable intensity, all the more impressive for the fact that you can't see his face and his voice is being converted through a ring modulator. Jacqueline Hill responds with an equally ferocious determination to stay, reminding you through her performance that to Barbara, Ian is just about the only thing she can rely on in the entire world right now. They're marooned together in a strange world, and the Doctor and Susan are aliens who've kidnapped them. She needs Ian, and the actress plays that perfectly.
And of course, the Doctor's response is to say, "Sounds good, let's go!" He's still not the Doctor as we know him. He is perfectly happy to skive off and leave others to take the brunt of his mistakes...and while some would argue that never entirely changes, at least later on he stops doing so knowingly and deliberately. This is a Doctor who is a coward, and Hartnell manages to play that effectively while still remaining sympathetic. And finally, Carole Ann Ford acts as the bridge between his alien morality and his human companions, instinctively grasping the importance of sticking together and helping those who help you. It's a brief scene, but it's rendered magnificently.
The second moment is when they do escape the city, and have to decide whether to go back to help the Thals. Here again, it's Ian who volunteers to do so alone, settling firmly into the mantle of hero at this stage in the series. (Yes, Sue is right. At this point, they really should have called the show 'Ian'.) Again, we see the Doctor entirely willing to agree with any plan that involves someone else taking the risk, and again we see Susan determined to do the right thing because at this point in the series, she's the Doctor's conscience externalized. But Barbara, this time, has calmed herself to the point where she recognizes the wisdom of Ian's plan. It shows that she's not hysterical or clingy, which the previous scene might have conveyed on its own; she's intelligent, sensible, and no more in over her head than everyone else is. Which happens to be a lot.
And the final moment is the cliffhanger which is delightfully experimental, playing with the audience's expectations almost before it's had a chance to form them. We're at the end of the fourth chapter of the serial, the Doctor and his friends have reached the safety of the TARDIS just like they did at the end of "The Firemaker", and it's time to be off. But just when the audience is assuming they know the structure of the show, and just when they're assuming it's all over, Ian drops the bombshell. They can't leave without defeating the Daleks. They have to get involved, whether they want to or not. And every moment of that realization is etched so perfectly on the regulars' faces that it's a better cliffhanger than any monster scare ever could be.
Ian, Barbara and Susan. I don't think that Doctor Who could ever have succeeded without them.