The Web of Fear, a 6-part story starring Patrick Troughton as the Doctor, was first broadcast in 1968. Only part one of the original story still survives on video, but the script of the entire story can now be read on these pages.
The story is a sequel to a previous adventure, 'The Abominable Snowmen'(1967). In the earlier story, a non-physical entity called the Great Intelligence used robotic Yeti slaves to take over a monastery in Tibet. The Doctor, Jamie and Victoria, helped by an explorer named Travers, defeated the Intelligence by shattering a giant glass pyramid, its physical link with Earth.
Travers brought a number of relics back from his Tibetan journey, including a (now lifeless) Yeti and a control sphere, a small metallic sphere from a cavity in the creature's chest, which the Intelligence used to control it. His interest in the workings of the Yeti's sphere led him to become an expert in robotics, and 40 years later, a much older Travers, now a Professor, manages to reactivate the control sphere. Unfortunately, the sphere promptly disappears as the Intelligence is awakened, and the way is clear for a second invasion of Earth, this time centred on London...
(The opening scene of episode 1 is a follow-up to the previous story, 'The Enemy of the World', which [SPOILERS!] ended with the villain, Salamander, operating the TARDIS with the doors open and being sucked out into the void. Patrick Troughton's dual role in this story, playing the Doctor and Salamander, also led to him being given a week's holiday during episode 2 of 'Web', hence the Doctor's absence during this episode.)
Click to read the script for: