Thea Ransome 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 " )