As with "The Dead Planet", this is an episode that's much more about the decisions that everyone makes in the episode, and the rationales behind them, than it is about the events. Not much actually happens here, any more than it did in "The Dead Planet"; the majority of the episode centers on whether or not the Thals will help the Doctor and his friends. More importantly, it centers on whether they have the right to convince the Thals to fight and die in order to rescue the Doctor from his own stupidity. The scene where Ian, Susan, the Doctor and Barbara argue about whether they should convince the Thals to go to war is really startling for a number of reasons.
The first is the way it shows a new side of Barbara. After only nine episodes, she's really established herself as the moral center of the group, and so it's shocking to hear her take the harshly pragmatic tack of pointing out that they can't spend the rest of their lives trapped in a petrified jungle with the Thals waiting for the Daleks to kill them. When Ian points out that she's being selfish, and she doesn't disagree, it's a side of her that we've not yet seen. (And it's also clever of the show to resist having "sides". This fight is the Doctor and Barbara against Ian with Susan as the mediator. We've previously seen the Doctor and Susan as a pair, and Ian and Barbara as another pair, but now we're forced to reexamine everything we know about the group.)
The second is the emotional intensity Ian brings to the scene. Again, you don't get companions like this anymore. He's seething with frustration at the situation he's in, knowing that he's about to ask people to die for him. "What victory are you going to show these people when most of them have been killed, eh? A fluid link? Is this what you're going to hold up to them and say, 'Thank you very much! This is what you fought and died for!'?" It's a brutal, scathing indictment, delivered with amazing passion. (Again, I hate to keep bringing up the Aaru version, but the same scene as done by Roy Castle and Peter Cushing has absolutely no dramatic tension. It's also telling that there's no deliberation on the part of the Thals afterward in the film, whereas in the televised version their decision is the centerpoint of the episode.)
And the third is that the Daleks are right. In the scene immediately preceding this one, the Daleks are watching the TARDIS crew with the Thals, and they say that the logical assumption is that the two groups will try to kill the Daleks. And it's completely true. The Daleks' paranoia and fear of all non-Dalek lifeforms, at this crucial juncture in their history before they became monsters, is absolutely proved true. It's really quite chilling, and carries some horrifying implications for later stories. Would the Daleks really be so aggressive if the Time Lords hadn't kept trying to unhappen their very existence?
But then there's the defining moment for the Daleks, the one that I talked about in the essay on "The Survivors". So far, they've been monsters that don't know they're monsters. But in this episode, they find out. They distribute the anti-radiation drugs, thrilled at the idea that they can finally escape their metal shells and their sterile city. Their five hundred-year long ordeal is over and things can return to normal. Even the Thals, their long-feared enemies, are within striking range, and the war can finally end...
And like Rappacini's daughter, the cure is poison to them. The radiation they had assumed was toxic is now their lifeblood. They can only survive on a dead world. What is survival to all other lifeforms in the universe is anathema to them, and they are damned to skulk inside their metal shells forever. Is it any wonder that their next move is to utterly lose their shit and set off a neutron bomb that will wipe out every living being on the planet that isn't a Dalek? For them, this is the point of no return. If they must be monsters, then they will be fucking effective ones, at least.
This is the moment where the Daleks are truly born. And because the Doctor really doesn't become the character we know him to be until he has some monsters to fight, you could argue that this is when Doctor Who is truly born as well.