The doors of the TARDIS open of their own accord just before it materialises, running out of control. On emerging, the travelers find the ship has been reduced in size and they are now only about an inch tall.
As tiny people, they stumble across a plot by a ruthless businessman,Forester, and his misguided scientist colleague, Smithers, to launch a new insecticide, DN6 — a product so destructive that it would kill not only those insects harmful to agriculture but also those vital to it.
Forester is willing to commit murder to ensure the success of his business, as civil servant Arnold Farrow discovers to his cost.
The criminals are brought to justice when the Doctor and his friends — hampered by the fact that Barbara is ill from the insecticide — tamper with the telephone in Smithers' laboratory, fuelling the suspicions of the local exchange operator, Hilda Rowse, who sends her police constable husband Bert to investigate.