Rose tried to gather some warmth from the fading embers in the crude fireplace. The Doctor was standing over by a blackboard, peering through the dim light at some chalked figures.

The place was little more than a hovel. A log cabin set in the middle of a white tundra.

There was an untidy mess of unidentifiable half-eaten food, scattered papers and old clothes, barely rags. A rickety old table and chair and a rough cot bed completed the scene. And the place smelled…

"'I'll show you the Universe' he says," said Rose, with heavy sarcasm. "This place is the pits. Where are we exactly — not to mention why?"

The Doctor waived his hand airily. "Siberia" he said, still focusing on the blackboard.

Using the edge of his sleeve he deftly rubbed out a line of figures and replaced them, writing carefully with the stub of chalk. He stood back to admire his handiwork.

Satisfied, he turned to face Rose." About 12 miles west of Ufa. August 23, 2019. "As to why, I think the answer is just about here…"

Rose turned at the sound of crunching footsteps approaching outside. The door was thrown open and a man staggered in with an armful of logs. He was accompanied by a freezing blast of wind and snow, which set the single lightbulb swinging, casting weird, elongated shadows inside the cabin.

The man dropped his logs and looked at them with a mixture of fear and suspicion. In keeping with the room, he was disheveled, dressed in rags and furs. Probably in his early sixties. "What do you want? There is nothing of value here. I have a shotgun…"

The Doctor moved quickly across the room, flashing his psychic paper at the bemused man. "Pavel!" he cried, delightedly. "I am Jerzy Smidkovic and this is Rosanna Tylarona. Didn't you get our letter — we are from the Institute." He pumped the newcomer's hand enthusiastically.

The man called Pavel peered at the Doctor's proffered 'credentials' "Ah the Institute," he said relaxing, slightly." I had no letter."

The Doctor tutted. "That is a pity. We are just passing through. I was asked to visit you. To see how your work is progressing"

"Ha!" spat Pavel "Progressing!" he nodded towards the blackboard. "Have you seen that pile of crap? I've been living like a hermit and getting nowhere for the past 18 months. I tell you the Theorem is dead — I'm going to resign and return to Moscow next week. In fact I was going to write the letter this very night"

The Doctor fixed Pavel with an intense stare. "Are you sure about that. Your work looks very interesting to me. Especially line 25…"

"Crap." Repeated Pavel. "A waste of time and effort"

"Line 25", said the Doctor, almost hypnotically.

Pavel snorted. He crossed the room and jabbed a finger at the blackboard. "That's the biggest problem of all-"he said. He peered at the blackboard, frowned, peered again and gave a double take. "IT BALANCES!" he screamed, making Rose jump. He danced around the room — actually danced." IT BALANCES!".

Pavel ran frantically to the blackboard, snatched up the stub of chalk and began scrawling furiously.

The Doctor inclined his head to Rose, indicating the door,"We'll be off then, Pavel. I'll tell the institute that your work is coming along, shall I? "

There was no response apart from the frenzied scratching of chalk on board. Then they were gone.

Back in the Tardis the Doctor was easing off his boots and replacing them with his comfortable plimsolls.

"He was a genius. The finest temporal mathematician ever. His theorem was a required text where I studied."

"So he became famous, then" said Rose, shaking the snow out of her hair. "I'm glad he got out of that awful place"

The Doctor shook his head sadly. "Pavel died in that cabin — about 6 weeks from now. The cold got him in the end."

Rose was appalled. "What happened to his work, then?"

"His notes went to the Moscow institute. Nobody understood them. They were just archived. About 350 years from now a research student at the Memory Prime orbital databank turned them up and the rest is history."

"What was so important about this theory?"

The Doctor started to operate the Tardis controls. "Well, without knowing it Pavel had gone part of the way to defining the Laws of Time, used by my own people. If you've got a spare five years I could teach you about the Blinovitch Limitation Effect — Pavel Blinovitch…a bit of a hero of mine."

"I'll take your word for it. Where are we going now?"

The dematerialization noise filled the chamber.

"Memory Prime" smiled the Doctor.

THE END

With acknowledgement to the film 'The Day The Earth Stood Still'.