The Doctor and his companions push the Tardis's emergency escape button and find themselves outside of Space and Time in a misty Nowhere that eventually resolves itself into a world of books. The trees in the "forest" they get lost in are actually enormous wooden letters. They are hunted by wind-up toy soldiers and attacked by mythical beasts. They keep bumping into Lemuel Gulliver- who talks to them in great chunks of 18th century prose. There is some rather hazy SF explanation for all of this, but essentially we've landed over the rainbow.
The first episode- all that running about in the mist- was levered in at short notice to fill a scheduling gap. Frazer Hines was off sick for the filming of much of episode #2 so an incident was improvised in which he loses his face, thus allowing him to be replaced with another actor for the duration. These last minute adjustments are among the zingiest elements in a story that is more than usually wild and witty. Such are the benefits of working under pressure in a culture of make do and mend.
I'm very fond of Troughton's Doctor, but I could wish the dithering was a little less voluble. Jamie and Zoe are sweet. Ever-so-Scottish Frazer Hines turns out to have been born in Yorkshire.