So, apparently, the actress who played Barbara got a couple episodes off so she could take a vacation.
That’s…not a bad deal, truth be told.
OK, so, Ian collapsed, but the Doctor can finally act like a doctor and quickly diagnoses the problem as poison, not disease like the Sensorites figured. A little thinking points to the water supply, something the Elders don’t touch, but they’re the only ones not getting sick.
Now, the Doctor can cure the sick Sensorites, including Ian, if he can be allowed back on the TARDIS, and while the First Elder seems inclined to trust the Doctor, the City Manager thinks the Doctor is trying to trick the Sensorites since that’s what happened the last time humans came by. After all, he figures Ian is faking it.
As such, even when the Doctor figures out what the cure is and asks it to be delivered to Ian, the City Administrator intercepts the vial and destroys it, figuring Ian will eventually stop faking it.
Besides, the Doctor also figures out where the poison is coming from and goes down there to that particular aqueduct. However, it turns out the Sensorites can’t see in the dark, are hurt by loud noises, and are really scared of monsters down there. Seriously.
So, when the Doctor goes down unescorted with just a flashlight, a still sick Ian and Susan follow him. That pleases the City Administrator, whose brilliant plan up to that point was realizing all Sensorites look alike and steals the sash of office from the Second Elder. That jerk figures the monster will eat the humans.
As it is, the Doctor finds some deadly nightshade down in the aqueduct. See, it is poison!
And then he hears weird noises. See, there is a monster!