This was the first story of season two. The TARDIS finally arrives in England in the 1960's, but there is a slight problem. The TARDIS doors opened accidentally while the ship was still in flight and everything has been reduced in size, including the travellers, who are now one inch tall. They do not understand this right away and think they have arrived on a world of giants.
Ian is in a discarded matchbox when it is picked up by a scientist named Farrow (Frank Cranshaw) working for a callous industrialist named Forester (Alan Tilvern). Forester's new insecticide--DN6--is too deadly to all forms of insect life to go into production. Forester kills Farrow to prevent this news from getting out. Ian and Barbara hide inside the victim's briefcase to avoid being stepped on and are brought inside the house. The Doctor and Susan climb a drainpipe to get in and rescue them.
They see giant insects die on contact with DN6 and Barbara begins to fall ill. They try to lift the giant telephone receiver to call for help, but this does not work, so they set a fire to attract the attention of the police.
The serial was only 73 minutes long and a bit simple. The set design received praise, the story and acting not so much. The fact that it was a four-part serial condensed into three did not help.
It was intriguing to see the Doctor involved in a standard SF plot--so standard, in fact, that it has been done hundreds of times, from Dr. Cyclops in 1940 through The Incredible Shrinking Man in 1957 to the entire Land of Giants TV series from 1968 to 1970, which was rather exciting thanks to Irwin Allen's production and the music of John Williams (who also created the music for Allen's Time Tunnel).
It is relatively easy to create giant insects and giant familiar objects through mechanical contrivance, which is so extremely difficult with high forms of life--compare the giant ants in Them! with all the plywood pterodactyls we've seen. The ancestor of all of this, of course, is Gulliver's Travels, in which Jonathan Swift (allegedly a distant relative) gave us both variations--a giant human in a Lilliputian world and a normal-sized human in a World of Giants.