Oh, please don’t keep talking on the twentieth century level. I’m talking about time travel!
Having escaped the French Revolution with his head (if not his fancy hat), the Doctor tries yet again to get Barbara and Ian back to the twentieth century. Given that they’re so close, just a few hundred years off, he attempts to “sidestep” into the 1960s with a simple frequency change on the TARDIS controls. And it works! They’re in England even. Of course, they’re about an inch high when they leave the ship, starting the second season of Doctor Who on the “Planet of Giants” (Story Production Code J), but hey, it’s a start.
Because the TARDIS door opens mysteriously upon materialization—a time Susan declares “the most dangerous moment” in the operation of the TARDIS—the “space pressure” exerted upon the TARDIS at this point shrinks the Doctor and his three companions, a fact they finally realize when they confront giant ants and earthworms and matchboxes.
The travellers split up upon arrival, with Barbara and the Doctor going one way and Ian and Susan the other. This particular grouping allows both Susan and the Doctor independently to come to the realization that they have landed on Earth in a shrunken form.
Indeed, this story presents Susan as a canny and calm time traveller, piecing together clues about their predicament, showing knowledge of TARDIS operations, clambering up corroded drain pipes, striking oversized matches, and even reminiscing with her grandfather about that time they suffered through a World War I zeppelin air raid together.
Until, of course, she screams and screams when she sees the dead giant ant and the not-dead giant cat. In her defense, it is a very giant cat.