The TARDIS materializes in England where the Doctor and company meet a girl named Isobel.  Her uncle is missing and the Doctor offers to help find him.  They go to International Electromatics, the world's major supplier of electronic equipment, where he was last seen.

The Doctor meets up with Lethbridge-Stewart who is now a Brigadier.  He works for the British branch of the United Nations Intelligence Taskforce (UNIT).  He confirms that Tobias Vaughn, the head of International Electromatics is involved in something shady.  They soon discover that he is, in fact, teamed up with the Cybermen.  The Cybermen use a hypnotic signal broadcast though circuits made by International Electromatics to launch their invasion.  The Doctor protects himself and his companions and the invasion is eventually defeated.