After returning from the Middle Ages, the Doctor relaxes by going fishing. However, he finds his relaxation cut short when he is called to investigate the destruction of a television mast, apparently caused by metal-eating insects. The creatures have been bred by scientist Professor McTurk and his daughter Catriona. They plan to use the insects to extort money from the British government in revenge for ignoring the Professor's talents. When the insects are released on an armyfiring range, their devastating attack on a tank battalion is witnessed by two foreign spies. The enemy agents trace the insects to the McTurks' farm, where they shoot the Professor. Honouring McTurk's dying wish, Catriona releases the insects to wreak havoc on the world. As the insects swarm toward Blackpool, the Doctor enlists Catriona's help to prevent them heading out to sea and the rest of the world. One thing can save the Earth now — Blackpool Tower!
Although the enemy agents are never identified as such, they are heavily implied to be Russian, being depicted as caricatures. As such, they address everyone as "comrade" — even people they are about to kill.
The Doctor notes that the swarm is travelling in an eastward direction and heading towards Blackpool. In the real world, however, Blackpool is located on the west coast of England.