When things go odd on the TARDIS, they go very odd. A basic cosmic catastrophe, the universe imploding or something like that would be an easy response to a short circuit aboard an antique Type-40. However, those sorts of happenstances are the sensible and rational subject of American Sci-Fi, whenever the Enterprise or Death Star decides to go wonky. On Doctor Who, its the whole universe that decides to go wonky around it.
En route from the French Revolution, the Doctor is once more trying to bring Ian and Barbara back home to 1963 England. When he's just about got it figured out, however, a malfunction blows open the doors of the TARDIS during the most dangerous part of the journey: rematerialization. On pushing the doors closed and completing the operation, everything seems fine at first. The discovery of a massive earthworm, ant, and box of matches convinces them otherwise. Spatial pressure within the TARDIS changed while its doors were open, resulting in the crew being reduced to a size of mere inches. Like every adventure with the Doctor, it would have been simple enough to hop back in the TARDIS and fix things. Unfortunately, Ian falls into the matchbox and is taken to the site of a grisly murder over an experimental insecticide... an insecticide that threatens the life of Barbara.
Once it was known that Doctor Who would outlast its first season and initial filming run, Planet of Giants was rushed into production. It is a serviceable jaunt into weird territories, cribbing from The Incredible Shrinking Man, gangster movies and ecological fears over insecticide use, but this deep into Doctor Who, it would be pointless to recommend either watching or missing it. As the debut of the second season, it displaced the original finale of the first season into the 10th story slot and became the penultimate story to a major shift in the series' cast. That shift, however, will have to wait for 2164 and the apocalyptic aftermath of an invasion by Britain's favorite malevolent aliens.