Character articles should not be plot summaries.
Thea Ransome was one of a team of scientists and technicians working for Dr. Fendelman at Fetch Priory, studying the enigma of "Eustace", an apparently human skull which had existed eight million years before archaeology had estimated humans had evolved. In actuality, they had found not a human skull, but the remnants of a dormant Fendahl Core. Thea worked alongside Fendelman, Maximillian Stael and Adam Colby.
In July 1977, when Fendelman activated his time scanner, Thea saw Eustace glowing and began to feel the skull's influence upon her own mind and clutched at her head. The next day, she could not remember what had happened. When Colby returned from walking his dog, Leakey, he told Thea, Max and Fendelman about a dead hiker he had found in the nearby woods. Despite her own objections, the others agreed that the body should be moved so as to avoid any impediments to their work. Thea pressed Adam as to whether he agreed, and he replied that it made little difference. Thereafter, Fendelman had a security team at the priory, and Thea though the restrictions they imposed - no one being able to come in or out without permission - were ridiculous.
Thea was more convinced at the viability of Fendelman's experiments than Adam. He labelled the time scanner an "electronic fruit machine", but Thea countered that "Fendelman's no fool when it comes to electronics", adding that he was one of the "authentic geniuses" of the field.
Thea secretively went to Fendelman's lab and turned on the time scanner. Again, Fendahl core exerted its influence over her. Adam found her there and attempted gain her attention, only succeeding after turning the machine off. He told her a scream had come form the kitchen and when they got there they found David Mitchell, the leader of the security team, dead. Thea observed that he had a blister on the back of his neck, but added that this might have been a birthmark. Adam questioned how she could be so "dispassionate" when the man was dead.
Thea suddenly collapsed and the Doctor entered. He warned Adam not to touch her and Fendahleen appeared over her body as she glowed. When this light stopped, she awoke. The Doctor suggested to Fendelman that he x-ray Thea's skull, but he replied that he gave the orders and took the Doctor prisoner.
Thea asked Adam why he hadn't called the police after the first death and he replied that he was ambitious, which he said was a weakness in anyone. Thea agreed, adding "particularly scientists." Thea told him to the phone the police now, but he found the line had been disconnected. Adam said "the place is surrounded by guards, we're beset by a wandering lunatic and we have a pair of corpses on our hands. And on top of all that, the telephone seems to be very dead. Thea, we're trapped". In reply, Thea said that "it was planned", and, when he didn't ask who by, she told him "I did". Adam was disbelieving, telling her she was "as sane as anyone here".
Thea sat quietly in the kitchen as Fendelmen told Adam his theory about Eustace — that it was a skull of extra-terrestrial origin. As they continued to argue, Thea told them she was tired and moved to leave. Fendelman told her she had perhaps been working too hard and said he would Stael look in on her later.
Thea went to find the Doctor to ask for his help, but discovered that he had escaped imprisonment. She crossed the corridor and entered the lab. Max followed her in. Thea explained that she had been looking for the Doctor, but he replied that it was of no matter and that it was "fitting" that she should be the "key" to his power — "the chosen one". He then pressed a cloth to her mouth and she fell unconscious.
Thea woke in he priory's cellars. Max told her that she was to be the "medium through which the ancient power of this place is focused" before injecting her with a hypodermic needle. He told her that the scanner had awoken "the power", that he had been watching her for some time, and through her he would "conjure and control the supreme power of the ancients". Thea called him a fool, and Stael replied that he would be a god. (TV: Image of the Fendahl)
When the Eighth Doctor travelled through the time fissure which was enhancing Fendelman's time scanner, he briefly looked out through the scanner and saw Thea shortly before her transformation. (PROSE: The Taking of Planet 5)
With Thea lying on a pentagram on the floor of the cellars, Stael's coven began their ritual. Eustace was placed on an altar and coupled to the time scanner. Thus, Thea was transformed into the Fendahl core. She began killing the coven members, beginning with Ted Moss.
The Doctor killed one of the Fendahleen, which was created from pure energy while the skull was restructuring Thea's brain. In the cellars, the Doctor freed Adam, who had been bound by Stael. The Doctor found Stael himself unable to move, having looked upon Thea's changed eyes. He killed himself with a gun the Time Lord handed him.
The core appeared to Leela and Jack Tyler besides a Fendahleen. Having looked upon Thea's eyes, Jack was unable to fire his gun. Leela knocked him down, took the weapon and, without looking, fired to kill the Fendahleen.
Using the time scanner to confuse it and salt to repel it, the Doctor was able to retrieve Eustace from the cellars and place it in a lead-lined box. The Doctor had rigged a controlled explosion to destroy the priory and so he had Leela moved to leave. The core appeared before them in the hallway of the priory, but they averted their gaze and walked through it. Once they were in the grounds, the priory and everything in it (including the Core, the Fendaleen and anything that may have remained of Thea's consciousness) was consumed by the flames of the explosion. (TV: Image of the Fendahl)