Within Fetch Priory, Dr Fendelman had a lab where he performed experiments using a time scanner in an attempt to examine the origins of Eustace, a seemingly-human skull that pre-dated the earliest known men by millions of years. While Adam Colby and Thea Ransome were also part of his team, it was only Maximillian Stael who was taken into Fendelman's confidence and allowed both knowledge of and participation in these experiments. As a result of his exemption from this, when Fendelman said there were things to consider after the body of a hiker was found in the woods near the priory, Colby asked if he was "breeding vultures in that secret lab of [his]".
When Colby went to the lab in search of Fendelman, the doctor entered behind him and asked his he was impressed by the machinery therein. Colby replied "I always say if you've seen one jukebox, you've seen them all." Fendelman then explained to him the purpose of the technology - to "see into the past", as Colby explained it to Thea.
With the Fendahl affecting her mind, Thea went to the laboratory and turned on the time scanner. Colby found her there and, only after turning off the machine, was able to stir her from her trance. As a scream had sounded from the priory kitchen, they both left in search of its source.
When Ted Moss stole into the priory, he met Stael in the lab and warned him about the arrival of the Fourth Doctor and Leela, saying that they had come to investigate. Stael said he would deal with them and told Moss to go.
Having told Colby about his work, Fendelman showed him in the lab the x-rays he had taken of Eustace. He explained his theory that energy was contained within the pentagram shown on the cap of the skull and that its release would be a signal to man's alien predecessors.
Thea again returned to the lab and Stael found her there. He told her it was "too late for all the meddling fools" after she said she was looking for the Doctor. Thea asked him to get out of her way as she was unable to leave, but he told her she was the "chosen one" and, placing a cloth over her mouth, rendered her unconscious.
In the lab, Fendelman explained to Colby that his work had been "a joy" after the latter noted that he had accrued over ninety-eight hours of the time scanner's use. The doctor then explained that he had reprogrammed the computer to give a visual interpretation of what the scanner picked up. It would show, Fendelman said, "the true genesis of homo sapiens."
As the machine was working, Stael entered and, threatening the pair with a gun, demanded that they turn off the scanner. Fendelman refused, but Colby did so. Fendelman asked why and Stael told him he was "not yet ready". As Colby mocked him, Stael said he would not warn him again. Colby said "you're going to kill us anyway, aren't you?", to which Stael replied "that depends on whether I enjoy having you worship me."
As part of his ritual, Stael connected Eustace, in the cellars, to the time scanner in the lab. In this manner, Thea was transformed into a Fendhal core.
While in Fendelman's lab, Leela said to the Doctor that she believed, given that one Fendahleen had been killed, the rest could be destroyed also. As the gestalt organism was not complete - Stael and a Fendahleen being dead - the Doctor agreed that it was possible to defeat it. He asked for more rock salt and Martha Tyler, who was with her grandson Jack Tyler, handed him the charms she had given to Leela and Jack, which contained the salt.
When Leela and Jack left, armed with rock salt cartridges for the latter's gun, the Doctor and Colby discussed the evolution of the Fendahl. A being developed which "ate life" and the Time Lords decided to destroy the Fendahl homeworld, Planet 5, and hide the fact from posterity. The planet had been placed in a time loop. They continued their discussion, with the Doctor affirming Colby's suggestion that the skull ahd come to Earth before the Time Lords had acted. The Doctor said, "what happened was this" - "the energy amassed by the Fendahl was stored in the skull and dissipated slowly as a biological transmutation field. Now, any appropriate lifeform that came within the field was altered so that it ultimately evolved into something suitable for the Fendahl to use." Colby asked if the Doctor was saying the skull created man. He said no, "I'm saying it may have effected his evolution." The Doctor said it would explain the dark side of man's nature, but that it was just a theory. Colby said it was a wild one, and the Time Lord replied it was more fun that way.
The Doctor offered the alternative explanation that "the Fendahl fed into the RNA of certain individuals the instincts and compulsions necessary to recreate. These were fed through the generations till they reached Fendelman and people like him", which Colby found more plausible. Or, the Doctor said, it would just be coincidence.
Martha Tyler brought a trolley loaded with salt in vials into the lab for the Doctor's use. He told her and Jack to return to their cottage. He asked Colby to give him and Leela enough time to reach the cellars before running the time scanner for two minutes. This would confuse the Fendahl and allow them to retrieve Eustace. After a further three minutes, a controlled implosion he had rigged would go off, so Colby must get clear.
Colby did as asked, running the scanner for the specified time and then leaving the priory and following the Tyler's to their cottage to shelter. With Eustace in their possession, the Doctor and Leela left and the priory was engulfed in flames. (TV: Image of the Fendahl)