The one where the Doctor plays environmentalist.
All the usual fun and frolics with perfectly normal household objects becoming perilous deathtraps are there, but they also accidentally stumble upon a plot by ruthless businessman Forester to launch a new insecticide, DN6, despite it being harmful to other life. A civil servant named Farrow investigates, but upon refusing to back down is murdered by Forester (told you he was ruthless). Forester then summons the creator of the insecticide, Smithers, and blackmails him into helping cover up the crime.
Barbara is made ill by the insecticide, but the others manage to foil Forester's plan by tampering with his telephone, causing the local telephone exchange operator to be suspicious and send her policeman husband along to investigate. He isn't murdered, and brings Forester to justice.
This story was originally produced as four episodes, but it was decided the original edit was too slow-paced for a season opener (and Carole Ann Ford's departure in the next story made it impossible to rejig the running order to have another story go first). Hence, episodes 3 and 4 were merged into a single episode using about half the material from each in post-production, making it the only three-part story in the first 23 years of the show's run.
The Irwin Allen series Land of the Giants bears a remarkable similarity.
This story provides examples of:
- Behind the Black: Ian and Susan don't notice the "giant" dead ants which are merely a few feet away from them (relative to their size) until the ants are shown onscreen. Similarly, after they find the first ant egg they marvel at it, then look down and find a whole bunch more.
- Corrupt Corporate Executive: Farrow, though it's never stated what position, if any, he holds. He could just as easily be some sort of venture capitalist.
- Definitely Just a Cold: Barbara tries to write-off her poisoning symptoms as some sort of minor illness.
- Eye Scream: The Doctor manages to have an aerosol can of insecticide (labelled "Highly Flammable") explode right next to Forester, who reacts by holding his hands over his face and screaming in agony, heavily implying some metal hit him in the eye.
- Failed a Spot Check: Barbara specifically tells Ian she got some of the sticky stuff on her hands, but later he doesn't know that she got poisoned.
- Foreshadowing: Ian ponders what kind of planet would produce large insects. Later this season, he'll find out.
- Green Aesop: Spraying chemicals everywhere, even with good intentions, can have unintended consequences.
- Worth pointing out that this serial was made just two years after the publication of Silent Spring, which kick-started the modern environmentalist movement and led to (among other things) the banning of DDT.
- Hand Wave: The reason behind the shrinkage of the Doctor and Co. is... Space Pressure.
- He Knows Too Much: Happens to Farrow when he crosses Forester. Nearly happens to Smithers but for a can of insecticide blowing up.
- Incredible Shrinking Man: Obviously.
- Little Old Lady Investigates: Hilda.
- Mega Neko: The TARDIS crew are menaced by a gigantic housecat. (In reality, the cat is normal sized but they have been shrunk so it is gigantic to them).
- My God, What Have I Done?: Smithers when he realises just how dangerous his pesticide is, especially after his cat dies.
- Obstructive Bureaucrat: Subverted. Forester thinks of Farrow this way, but Farrow is actually in the right.
- Poor Communication Kills: Barbara gets poisoned and gets progressively more ill as the serial goes on, but hides it. A lot of people seem to think this is a plot hole, but it's really just traditional British culture, stiff upper lip and all that. Everybody's already trying to get back to the TARDIS, so she feels explaining the poisoning would hurt them unnecessarily.
- Pyromaniac: The Doctor seems very enthusiastic about causing a gas explosion.
- Ripped from the Headlines: The story is rooted in the recent release of the book Silent Spring, which exposed the damage DDT was doing to bird populations. The story concerns an ecologically-devastating pesticide.
- Smart Ball: Susan, with her observations about the nature of the insecticide. May simply be her failing to hold the Idiot Ball for once, given her Informed intelligence in the pilot and a few of her later stories.
- Two Scenes, One Dialogue: When they finally realize that no, they're not at the World's Fair.
- Visible Boom Mic: Around the 4:30 mark of "Crisis" a boom microphone can be seen entering the top of the frame for a few seconds. This happens again around the 14:14 mark.
- Well-Intentioned Extremist: Smithers believes initially that his pesticide is the only way to avoid world starvation.
- Yank the Dog's Chain: the uncontrollable TARDIS lands by chance back on Earth in Ian and Barbara's home period (early-60's England), but they're an inch tall and have no way of getting back to normal without taking off and getting lost again.