Man, don’t mess with the Doctor. He will give you such a serious talking-to…
That’s more or less what happens after Susan left with the two Sensorites. The adults chase the trio down a corridor, and while Ian and Barbara can’t forbid Susan from going with the aliens, the Doctor can tell the whole lot of them off, and he and Susan have what may be their first argument ever over whether she was being brave or stupid by trying to go with the weird aliens who communicate long distances by pressing a small disk to their foreheads.
By the by, there’s a scene in this episode where the Sensorite leaders down on their planet comment on how ugly the humans are, but then again, maybe Sensorites are ugly from the human perspective, so maybe they shouldn’t talk.
What follows is a negotiation. The Doctor, Ian, Susan, Carol, and John will journey down to the planet. The Doctor, Ian, and Susan will meet with the Elders and try to work something out. All the Doctor wants is the lock to the TARDIS so he can get back inside and freedom for the ship the humans have been stuck on for a while.
As it is, the First Elder seems inclined to trust the Doctor. He seems wise, he didn’t threaten anyone, and it turns out Sensorites can be stopped by turning off the lights, a fact the Doctor did not use to his advantage.
They get really lost in the dark? These guys suck as a species.
Oh, it also turns out the TARDIS’ lock is the most sophisticated piece of equipment the Sensorites have ever seen.
So, it turns out that this was not the first human ship to visit the Sense-Sphere. The previous one had tried to steal some precious metals, inviting more ships for what might be called “raping and pillaging,” and after they were expelled, they left some kind of plague behind that is killing the Sensorites all over the place. Since the Doctor seems to abhor violence, he seems like a guy who can maybe help.
Too bad some of the other Elders are plotting to kill the whole party. Only the timely intervention of the #2 guy stops the #3 guy from disintegrating the Doctor and his party. But there may be other ways to bring about some death.
You know, like when Ian somehow contracts the illness and collapses on the floor.