Exiled to Earth by the Time Lords as punishment for stealing a TARDIS many centuries ago, the Doctor has taken up residence in the Carlton Grange Hotel in London. Not far away top medical brains are huddled around a newspaper with a picture of the Doctor on its front page, supporting the headline, "Time and Space traveler residing in London".
Dr Rowan Cartwright calls the Carlton Grange Hotel and tells the Doctor that he may be of invaluable assistance to him and his colleagues at the Medical Research Center. A thorough examination is arranged for the Doctor, who can't think what Cartwright and his fellow doctors are going to learn about his time battered body. He tells them that he is not constructed in the same way as an Earth-man. Cartwright then remarks that there is much they can learn from his tissues, and that they are the same as a Human's. As Cartwright bares the Doctor's chest for examination, an incredible arrowhead mark is revealed, that Cartwright calls the mark of terror. He then collapses and passes out into a deep coma. Dr Who explains that it is actually The Mark Of Blenhim, and tries to work out why it should have such a devastating effect on Cartwright.
The Doctor gropes back through the past, and recalls that several hundred years ago, a master race called the Blenhims wreaked a reign of terror on all the planets in their solar system. He says that he had heard some refugees fled through space toward our galaxy, and surmises that a few space refugees landed on Earth in the eighteenth century and it could well be that Cartwright is one of their descendants. The Doctor's hypothesis continues with him stating that The Mark Of Blenhim, which spelled terror for Cartwright's ancestors, and left its mark on their minds, could have been passed down through the generations to Cartwright himself. The other doctors have tried everything they can to bring him round, but the Doctor tells them that they wont be able to until the horror of the Blenhims is erased from his memory. The Doctor then asks the doctors if they have any thought-transference equipment, and some time later he and Cartwright are hooked up to such a device, with the idea being that the Doctor relating his encounter with the Blenhims, passing his own thoughts into Cartwright's head, may help shake him from his coma.
The Doctor remembers a huge rally of vicious dark robed, hooded warriors, saluting an all powerful leader. He remembers materializing the TARDIS in an underground car park full of futuristic vehicles, and seeing hundreds of these hooded warriors making their way to a stadium, he then sneaked into through a window. The experiment is said to be going fine with Cartwright receiving all of the Doctor's thoughts.
The Time Lord finds spare robes and hoods, disguising himself, so as the Blenhims would never know they had a stranger in their midst. The stadium is filled to capacity, with everyone dressed in the same way. Their leader then speaks from a stage, saying that he has some important news regarding their plans to dominate all the planets in their galaxy, telling them that at this very moment their missiles were being launched against all the planets that had refused to accept them as their masters. He adds that the Blenhim reign of terror had begun.
The Doctor's recollections are now having a terrifying affect on Cartwright, who is said to be close to breaking point, but still the experiment continues in the hope the Doctor's story will shake him out of it. Back in the stadium, the Blenhim Leader has a large video screen that is showing all those gathered, Blenhim rocket-craft strafing the defenses of many of the surrounding civilizations, and inflicting terrible damage. Not even the most isolated dwellings on remote planets escaped their vicious terrorism. The devastation rendered millions all over the galaxy homeless, with space refugees fleeing for their lives. The mention of the space refugees really upset Cartwright with the other doctor's concurring with the Doctor, that he has an inborn fear of the Blenhims passed down by the forefathers who suffered at their hands. The Blenhim Leader's film show was over but the Doctor knew that the warmongers among them had to be stopped, so he moved toward the leader's platform and spoke into a microphone in front of the entirety of all those seated in the stadium. The Doctor is called to the Blenhim Leader, and said, "I question you're right to govern us. I question your power". The Leader tells his guards to seize the Doctor, but he is reminded that it is the right of all Blenhims to challenge the leadership. The Doctor boasts that his powers are greater than the leader's and he wishes to be put to the test. The Leader agrees to the contest, but tells the Doctor that when he proves himself superior, he will have this wretch torn limb from limb. The Leader inflicts wounds upon one of his own kind, using only his eyes, but the Doctor calls this a cheap trick, and says that he will treat them to something of a more sensational order.
A monument that stood in the center of the stadium was then atomized seemingly by a beam that shot out of the Doctor's outstretched hand. In reality however he had used the laser ring he had taken from a Cyberman in a previous encounter. It was enough to turn the Blenhims loyalties and for them to demand Dr Who become their leader. Again taking the microphone, Dr Who commanded that his orders be transmitted to all fighting forces attacking other planets, and that they should all fly due east, something that because of their obedience to their leader causes them to plummet into the sun.
The Doctor tells the assembled doctors and an awoken Cartwright that this was how he destroyed the Blenhim Master Race. Cartwright says that knowing Dr Who had destroyed the Blenhims centuries ago has put his mind at rest, with the Doctor himself concluding that there was no point in fearing a race that no longer exists.
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