Due to a glitch the Doctor and Companions end up small. Very small. As in, smaller than the typical insect. But they don't realize this at first, as they are wandering through a landscape of strangely large but familiar looking objects. Such as a box of matches the size of a car. After awhile they realize they aren't trapped in an oddball amusement park, but instead they have been shrunk and the problem for them is how to return to "normal" size.
To make matters interesting the place they land was the home laboratory of a chemist working on a very potent insecticide. One which is especially dangerous to the humans in their extremely tiny size, as Barbara learns when she touches some of the insecticide.