While Barbara is meeting her first Dalek (although we don’t get to see it) at the end of Part 1, the Doctor, Susan, and Ian eventually return to the point where they entered the city. When Barbara doesn’t turn up, they go in search of her. Along the way, the trio comes upon a room containing monitoring instruments that appear to be in use; among these is a Geiger Counter, clicking rapidly.
So now they know all about the dangerously high radiation levels they’ve been wandering around in for the last two days and understand the significance of the ashen soil, the dead and crumbling forest, and this apparently empty city. They also realize why all of them have been feeling so strangely tired.
The Doctor posits that a neutron bomb would cause such devastation, but leave the buildings intact. I remember there being talk of neutron bombs back in the 1980s, but didn’t realize that the idea of such a nuclear weapon was around in the early ’60s. Whatever type of bomb caused this, it’s urgent that they hurry back to the Tardis and go to another place and time where they can be treated as soon as possible for radiation sickness.
Ian mentions that they must find some mercury, and the Doctor makes a confession: there’s nothing wrong with the little fluid link gizmo; he nobbled the Tardis just so they would have to come and explore the city. But now that he’s in danger, he wants to go and leave Barbara behind.
Back in the Stone Age story, I said that this version of the character is more like Dr. Smith than any Doctor I’m later familiar with–and here’s another example. But he isn’t as camp.
Susan and Ian refuse to go without Barbara, and Ian has the fluid link, so it’s useless for the Doctor to go back to the Tardis alone.
The argument turns out to be moot in any case. Before they can go anywhere, they are surrounded by oversized, xenophobic, metal pepper-pots that hysterically shriek “Exterminate!“