The TARDIS is sent out of control, and when it materialises on Earth it has shrunk, and so have the occupants who are all now around 1 inch tall.
The Doctor and his companions stumble across a plot by a scientist, Professor Smithers, and a buisnessman, Forester, to launch the new insecticide DN6. DN6 does not just kill insects that are harmful to agriculture, but also those that are essantial.
A Government agent, Farrow, discovers the plan but is murdered before he can reveal it. The minaturised Ian and Barbara witness the murder.
Despite the fact the being minaturised leaves the Doctor and his companions to the mercy of plugholes and giant matchboxes, the Doctor manages to create an explosion and also rouse the suspicsions of the local police who call round to investigate.