The Doctor, or the man calling himself the Doctor, shrugged and turned away to inspect his calculations chalked across the dungeon walls.
"I can live with that."
Anna scowled, but just wasn't sure enough of her ground to pursue the point. She had heard of the concept of regeneration, but had always assumed this meant the Doctor would revert to a younger version of himself. This ascerbic and rather rude individual was not what she had been expecting.
"What is all this anyway?" she asked grudgingly, indicating his mathematical graffiti.
"Well," he said, still contemplating it. "Before I got unexpectedly swooshed up and dumped down just outside the fort, I was conducting a little investigation into the disappearance of a young man. Unfortunately the guards took my instruments away from me while they were reading me my rights, or rather lack of them, but I remember a lot of the readings and I've just been figuring the ramifications."
"And?"
The Doctor pointed. Anna looked down into a dark corner of the cell and saw a heavily underlined solution at the end of the vast and incomprehensible series of formulae.
"Four."
He took another appraising look at his work and nodded with confidence.
"Yes."
"Four?" She looked round at him incredulously. "Is that it? After all this? What's it supposed to mean?"
"Quite a bit, considering that figure would normally be at least one point three six times ten to the power of seventeen. Rather worrying, considering it's a measure of the integrity of reality."
"The integrity of... what?"
He gave her that curving, lopsided smile.
"Oh, yes." He stamped his foot a couple of times on the floor. "Feels solid enough, doesn't it? So does thin ice, until it cracks beneath you. We're just a tenuous membrane away from total collapse. Not of matter or energy, but of the very stuff of existence, the thing that allows matter and energy and all physical laws to exist. Chaos, death, darkness. Not a pretty picture." He looked around pensively. "Either that or I've made a mistake, but that's not very likely."
Anna had been listening to him in gathering horror. The certainty that he knew exactly what he was talking about grew with every word.
"You mean... the end of the universe?"
He looked at her scornfully.
"Don't dramatise." He wandered off to go and peer down into the oubliette. "No, no, the effect would be very localised. End of the world. Well, the star system."
"What are we going to do?"
"Don't know." He lost interest in the oubliette and surveyed the rest of the cell. "But we should probably think about escaping from this dungeon at some point."
"Ah." Faced with a problem she could get to grips with, Anna recouped some of her usual cool. "Tell you what, I'll call the guard, tell him you've fallen ill, fainted or whatever, then when he comes in I'll bash over him the head with that stool, we'll put you in his uniform, and you can just march me out pretending you're taking me off for questioning or something."
The Doctor stood and let her finish speaking, but she had never seen anyone look so resolutely unimpressed.
"Yes. Well, one should never neglect the classics of course, but I think we'll try something a little less Buck Rogers."
His gaze swept every corner of the cell.
"Anna. Make yourself useful and unwind the chain from that bracket."
She took a deep breath, knowing this wasn't the time for an argument about his assumption that he was in charge around here, and did as she was told. The long iron bracket under the stone bench along the wall was wound around with a continuous length of chain - immoveable if somebody had been attached to any of the manacles linked into it, but as it was she was able to unwind it and drag it clear, leaving her with a tremendous bulk of loose metal that she couldn't even lift. She glanced over too see the Doctor dragging a great chunk of loose stone across the floor, pressing it up against the door.
"What are you doing? How are we supposed to get out now?"
"More to the point, how are they going to get in?" He looked over to check her progress. "Oh, good. Now throw it over the ceiling beam."
Anna looked up, and for the first time noticed the huge wooden support, which looked like an entire tree trunk roughly hacked into an approximate square, running across the breadth of the cell. The gap of a few inches between it and the ceiling were plenty to allow her to toss the end of the chain up and over with a single well aimed throw. She pulled the dangling end down and waited for further instructions.
"Now take the other end," he puffed, heaving another great cube of rock across the flagstones, "And tie it to the bar in the window."
Still flummoxed as to what he had in mind, she did so, looping the chain around the single horizontal iron bar that blocked the little window, tying it with the best knot she could manage with the cumbersome material, and reinforcing it with strips of hessian from the cell's rudimentary bedding bound tightly about the links to prevent slippage. She looked over at the Doctor, and found him wrapping his block of stone in an elegant chain parcel, adding in neat little hessian bows just as she had done.
"What's going on in there?"
The guard's voice, accompanied by the sound of a spear haft hammering against the door.
"Mind your own business," returned the Doctor, not looking up.
The bolts were drawn back sharply, and the door was thrust inwards, only to collide and jam against the lump of rock the Doctor had pushed against it. He looked up to survey the contraption they had just put together. The chain led up from the block at his feet to the beam, then drooped loosely down on the other side, trailing across the floor until it rose up to where it was tied to the bar in the window.
"Good."
A single vigorous shove with his foot pushed the rock over the edge of the oubliette where it plummetted out of sight. Anna watched the chain rattle over the beam, in a whiplash drawing up the slack on the floor the other side, and with an explosive impact like a rifle shot ripping the bar clean out of the window. A lethal iron missile, it hurtled across the room to embed itself four inches deep in the ceiling beam.
The Doctor shook his head as if in wonder at the success of his own plan.
"I am such a genius."
As more guards arrived in the corridor outside, hammering and shouting at the door, he ran across the room and clambered up, squeezing out through the window and onto the muddy ground beyond. Anna ignored the helping hand held patronisingly out to her, and scrambled up after him.
It was dark now, and raining steadily. The Doctor pushed his hands into his coat pockets and looked resentfully up at the sky.
"Oh, perfect."
"Come on!"
Like a dog straining at the leash, Anna trotted several steps ahead and looked back hopefully, willing him to get a move on and follow her. But he raised no more than a brisk walk.
"No hurry. They can't track us in these conditions, and no one's going to be rushing after us on a night like this anyway. Hmm." He was looking interestedly around at the curtain wall, dimly visible in the gloom, which looped out from the fort and enclosed a densely packed cluster of single storey thatched buildings. "Now, that's new. Last time I was here the fort was the only building for miles around."
"Look, we'll head for the Tardis," said Anna. "It's up on that hill, hidden in some trees."
"I know where it is. True, it's been a while but I can still remember where I parked it... oh."
He had been heading for the hill as she suggested, but something caught his eye and he made a ninety degree switch in direction, towards the dirt track that led back to the gates of the enclosure.
"Not the road!" Anna called after him, horrified. "That's the first place they'll look for us!"
"Won't be a moment, I'm just going to give this gentleman a helping hand."
She trailed after him, barely able to make out the figure of a man on the road struggling to shift a wooden handcart, one wheel sinking ever deeper into the gurgling mud.
"We've got to go," she insisted. "We don't have time for this."
"There's always time to be helpful."
Anna felt like she would burst with frustration as she watched him brace himself against the rear of the cart.
"That's so..."
She stopped herself. The Doctor looked up with an amused flicker of an eyebrow.
"Typical of me?"
Shaking her head in disgust, she took up position next to him, and with a synchronised heave they drove the wheel clear of the mud. The cart's owner looked round, relief flooding his features.
"My thanks to you. Both of you. You'd better come with me now. We have to get under cover."
The Doctor held out his hand to catch the rain.
"Why? It's only a shower."
The man stared at him blankly as if this were the most incomprehensible remark imaginable.
"It's when it rains that they come," he said, clearly hesitant about telling them something so obvious.
"When who comes?" asked Anna.
This bewildered him still further.
"Skypigs!"
He shook his head in disbelief at their obtuseness and dragged his cart away at full speed towards the fort. Anna and the Doctor exchanged glances.
"Skypigs?" she repeated.
They both looked up at the empty night sky, needles of rain splashing down on their upturned faces.