Following a malfunction on the TARDIS console and the bleating of a klaxon indicating something is amiss, the Doctor insists the fault locator shows nothing is wrong and it is safe to venture outside. He leads his companions Ian, Barbara and Susan to the world beyond and within minutes they find a dead giant earthworm followed by a large deceased ant. They seem to have died immediately. After some deduction the travellers realise they have arrived on Earth but have shrunk in size to about an inch.
Ian is investigating a discarded matchbox when someone picks it up and he is hurled around inside. That someone is a government scientist called Farrow. He is met by a callous industrialist named Forester to tell him that his application for DN6, a new insecticide, has been rejected. In reality DN6 should not be licensed: it is far too deadly to all insect life. When they fall out over this news, Forester shoots Farrow and leaves him for dead on the lawn.
The Doctor, Barbara and Susan hear the gunshot as an enormous explosion, and head for the house. They find Ian unhurt near the dead body and surmise a murder has taken place but can do little about it. They are determined, however, to ensure the murderer is brought to justice despite their microscopic size. While avoiding a cat, the travellers get split up again with Ian and Barbara hiding in a briefcase. The giant Forester returns to the lawn and collects the briefcase, taking it inside to the laboratory. His aide, Smithers, arrives and suspects him of murder, but does not report him for fear of undermining the DN6 project to which he has given his life.
The Doctor and Susan scale a drainpipe to gain access to the house and locate their friends, braving the height as they go. Meanwhile Ian and Barbara examine the laboratory and encounter a giant fly, which is killed instantly when it contacts sample seeds that had been sprayed with DN6. Barbara foolishly touched one seed earlier and soon starts to feel unwell. Nevertheless, attracted by Susan’s voice in the reverberating plughole, the four friends are reunited.