A/N: There are quite a few scientific explanations and stuff in this chapter, which is mostly things already learned in Doctor Who. There is some action at the end, though, so do read it, and please review! Thank you!

The two men stepped through a large set of double doors. Doyle sucked in a breath, gazing upwards at the large room he found themself in.

The room was shaped and furnished similarly to the console room, with round, orange-gold walls and coral struts. Looking around, Doyle could see many levels of metal floors stretching up to the heavens, and to further than he could see down below. All the levels were connected by a single spiral staircase which seemed to go on forever.

But the most remarkable thing about the room was not the size. It was the fact that every single floor was lined with rows upon rows of wooden bookshelves, each stuffed full of books. The shelves were surrounded by even more books, overflowing onto the ground and every perceivable surface. In the middle of the floors there sat several armchairs, sofas, large cushions, and other such comfortable places to sit, so that one could read the thousands upon thousands of books which stood upon the many bookshelves.

Despite the metal walls and floors, the whole space had a relaxed, cosy atmosphere, an effect which was helped by the noise of a crackling fireplace somewhere in the background, probably on another level.

Doyle walked over to the closest bookshelf, running his fingers gently along the row of leather-bound spines. He noticed that the titles of the books seemed to be in several languages, some of which he was certain were not from Earth.

He turned back to Sherlock. "This is amazing," he breathed, still marvelling at the sheer expanse of the space. "This must surely contain every book in the universe!"

"I doubt it, somehow," Sherlock said, moving away from the door towards the staircase in the middle of the room. "But it certainly has a lot of them."

Doyle managed to tear himself away from the bookshelf to join Sherlock by the staircase. "You said you could show me different universes?" he asked.

"I said I could show you the multiverse theory," Sherlock corrected him. "Come with me."

He took off up the narrow staircase, taking the stairs two at a time. Doyle's long legs easily kept up with him as they went around and around and around, bypassing many levels on their way up.

Sherlock didn't know for certain where he was going, or indeed if he was going to the right place. But he had a feeling in his heart, the sort he never would have trusted only a few days earlier, but since he had come into contact with the TARDIS he was becoming more open-minded about such things.

Some time after either of them had lost count of the floors, Sherlock stopped. He emerged onto a bookshelf-bordered level which seemed unremarkable when compared to the others, and headed towards a shelf to his left. His fingers wandered along the row of books until he came to a promising-looking one in English. "Here we go," he said, lifting the heavy book off the shelf and carrying it over to a table nearby.

Doyle leaned over Sherlock's shoulder to get a better look at the book. It was called 'The Multiverse Theory in a Nutshell', by a man called J. Harkness.

Sherlock cracked open the front cover and flipped through pages until he found the introduction. He pored over the page, eyes running over the small print until he found what he was looking for. "'In order to understand the theory of multiple universes, you have got to think in five dimensions'," he read out. "'Think of it like this: having no dimensions is a single dot. Just a dot, not going anywhere. One dimension is an infinite row of dots, or a line. Just one line, going infinitely in two dimensions. Two dimensions is an infinite number of lines, which make a plane. And three dimensions is an infinite number of planes, which make space.

"'Now, if you're able to read this book, I imagine you're familiar with three dimensions. It's here that things get a little harder. If each dimension is an infinite number of units of the previous dimension, then the fourth dimension must be an infinite number of spaces, all tacked onto one another, one after the other. This is what we call time. If there was only one space, we would all be stuck, not doing anything. With infinite spaces, we can experience the same space in many different instants, even in one second.

"So four dimensions can explain everything in our universe, right?'" Sherlock scowled a little at the rhetorical questions – he hated those in books – but continued reading. "'But what if we go one further? Imagine an infinite number of spaces, each with an infinite number of times. In other words, an infinite number of universes, all stacked on top of one another. This, my friends, is the fifth-dimensional theory: that our universe is only one of many in existence.

"This can be hard to comprehend, I know, but think about it. Think about where you are in life, and all the little things that had to happen to get you there. All the little coincidences, all the people you had to meet, all the buses you had to catch, to be there now, reading this book. Now imagine what might be different now, if even one of those things hadn't happened. You might be an entirely different person, with different experiences, different memories, different life choices.

"This, my friends, is the basis for the multiverse theory. The theory states that every time you make a decision, another universe is created in which everything is the same up until the moment you made the decision, and you made the other choice. Now, some of the decisions are large ones, like the decision to blow up a country or invade a planet. But some of these choices are much smaller, such as what coloured shoelaces to wear, or whether or not to choose a salad for your lunch.

"Imagine that, if you will. Infinite universe, infinite possibilities. Infinite versions of you, with different heights and races and hair colours and numbers of limbs. The possibilities are quite literally endless. Now, chances are, you're feeling pretty disbelieving right now. And who can blame you? You've picked up this book, and now a man's telling you that there is more of existence than you could ever have imagined. But what if I told you that there is recorded proof of this phenomenon? I wish I could go into more detail, but this is only the introduction. You'll have to read the rest of this book to find out.

"I hope you enjoy the rest of this book, and travel to lots of alternate universes, blah, blah, blah. See you around! By Jack Harkness.'"

Sherlock finished reading and looked up at Doyle. "Infinite universes," the latter breathed, staring back at Sherlock. "Fascinating."

"I agree," Sherlock said. "If it is true, that is."

Doyle said nothing, but took hold of the book from Sherlock and turned it around to face him. He turned to the back of the book, flipping backwards through the pages until he found the index. He quickly scanned the pages until he found the P section. He ran a finger down the column of tiny text until he found the entry he was looking for. "'Proof'," he read out. "There are three options: theoretical, inconclusive, and definitive."

"Difficult choice," Sherlock said dryly.

Doyle nodded, and ran his finger across the page to the number displayed. "Page three hundred and ninety-four," he muttered, turning back to the page in question. "Aha!" he exclaimed, cracking a smile. "I think we have it. 'Definitive proof," he read out loud. "Of the three kinds of proof of the multiverse theory, this is by far my favourite. Coincidentally, it is the one type of proof which is totally ignored by non-believers who would prefer to stay ignorant and in their own universe.

"The most obvious type of definitive proof is, of course, travel between universes. While this is occasionally achieved, it is usually by accident, and is strongly advised against by experts (not that there are many of those around). A memorable instance of trans-universal travel caused the Battle of Canary Wharf on Sol 3 (also known as Earth) in 2007, during which countless humans were murdered as a result of trans-dimensional travel by two alien species.

"It should be noted, however, that this is an isolated case.' Well, that is surely a good sign," Doyle commented. Sherlock raised an eyebrow at him over the book, and he rolled his eyes and kept reading.

"'Other cases include the infamous journeys of Braxos and company between their universe and one that was five years ahead of their own. They reportedly used information from the future universe to become rich and famous in their own universe, until the breach they had made between the universes expanded and almost caused both universes to collapse. After fixing the breach, all members of the company were then imprisoned in the Stormcage facility until their deaths.

"'(For a further discussion on the potential dangers of trans-universal travel, flip on over to page five hundred and three.)' We should look at that as well," Doyle added to Sherlock. "It could be important."

"Good idea," Sherlock nodded. "If alternate universes are involved in this somehow, we should be aware of the risks, if there are any." He took charge of the book from Doyle and turned to page 503. "'Dangers'," he read out. "'Here's the thing about trans-universal travel: it is dangerous. End of story. Whenever a ship, person, object, anything goes from one universe to another, it creates a crack in the surface of the dimension.

"Imagine it as a pane of glass. If you hit a window with something, a tiny fracture might appear. One small crack is manageable. Nothing to worry about, you think. I'll fix it later. But if things keep hitting the window, the crack will get larger and larger as time goes on. More crack will splinter off the first one, bleeding out from the first one like a spider's web. Each hit doesn't seem like much, but they all contribute to the larger cracks. And eventually, the whole window will shatter in thousands of pieces. Then it's gone, and you need to buy either a new window or be cold.

"That's what travelling between universes is like. It was never meant to happen, so every time someone travels between two universes, it put a crack in the barrier between them, which gets larger and larger until eventually both of them crumble into the Void. You can't just go and buy yourself a new universe. Not easily, anyway.

"Now, you may be wondering what the Void is. The answer is simple: it is nothing. Imagine all universes in existence as being stacked on top of one another, like an infinitely large stack of pancakes. The space between the universes, like the syrup or butter between the pancakes, is often known as the Void. Some species call it Hell.

"There is nothing there. Absolutely nothing. No space, no directions, no sense of the passage of time. If a person of any species manages to come back from being in the Void, they will not be the same. End of story. And the idea of oblivion is terrifying to sane people, which is why all species fear the Void, even if they do not properly know about it. Even one person could create a fracture in the barrier between their universe and the Void, and could cause everything to crumble. All the planets, all the creatures, all the stars and black holes and nebulas, would be gone. All the history would be forgotten.

"On a more cheerful note, this is very unlikely to happen. Travel between universes is both uncommon and difficult, and so there is no real danger that you will be crumbling into the Void any time soon. Probably.' That's the end of the chapter."

Sherlock closed the book with a dumbfounded expression. He looked up and locked eyes with Doyle, who was staring right back at him. It would have been evident to anyone watching that both men held the same disregard for personal space, so close together were their faces over the book.

"How would this apply to our situation?" Doyle asked eventually, drawing back and standing upright. "If multiple universes do exist, what does that have to do with us? Why did this machine tell us to look it up?"

"Firstly, it's not strictly a machine, it's a consciousness inside a machine," Sherlock told him, also standing upright. "Secondly, Mr Harkness said that there were infinite possibilities of reality within the universes. What if there is a different universe in which I only exist in books you have written?"

Doyle's eyes began to light up. "And that universe might be the one with which your friends are familiar," he completed. "A totally different universe, which is why they believe you should not exist."

"Exactly!" Sherlock told him. "A universe in which you're famous, and I don't exist. That sounds terrible, to be honest."

"I don't see anything wrong with being famous," Doyle commented. "It sounds quite nice, actually. I might be more successful at writing than I am at ophthalmology in that universe."

"So if they come from that universe," Sherlock continued, "then they must have travelled from that one to ours without realising. Would that be possible?"

"A few hours ago, I would have said that this whole conversation would be impossible," Doyle pointed out.

"Good point."

"More importantly, if they have travelled trans-universally, even without realising it," Doyle said, "surely they will have created a fracture in the barrier between the universes. Which means that…"

"…both universes are damaged," Sherlock finished. "They could both crumble into the Void."

"Inevitable oblivion," Doyle said quietly.

"We should warn them," Sherlock said suddenly, dashing towards the staircase in the centre of the room. Doyle picked up the large book and tucked it under his arm, before following Sherlock, his long legs easily keeping up with the detective's pace.

-o0o-

John and Harry hurried through the round door, guns at the ready, to find themselves in an empty corridor.

Harry froze, frowning. She exchanged a bemused glance with John. "This is definitely the right door?" she whispered.

"I'm sure it was," he whispered back, turning around to look at the door they had just come through. Sure enough, it was scorched and battle-worn on this side, and there was a small dent on one side.

"There was supposed to be a room on this side," she told him. "Even on the ship's plan, this door led to a storage room."

"I know," he murmured, squinting down the corridor both ways. "Where are all the Daleks?"

"I don't know," Harry muttered. "And I really don't like not knowing."

"Why are we whispering?" John asked.

Harry blinked. She straightened up out of the guarded position she had been standing in. "No idea," she said at normal volume. She pointed to the right with her gun. "Shall we go, then?"

"May as well," he nodded.

They set off down the corridor together, keeping in step the way they always did. They chose corners at random, pointing around them with their guns beforehand. There was no specific path they were following.

As they walked, John became more and more uneasy. The corridor hadn't been on the plan at all, and neither had the adjacent corridors they were now going down. He was sure they were walking into a trap. Of course, he had known before that they were going into a trap, but they had thought that it was behind the damaged door. Now he knew there was a trap, but they didn't know where it was, which made him all the more anxious.

Harry, meanwhile, was thinking about the Doctor. Goodness knew where he was, and what the Daleks had done to him, if he was even still alive. The last thing she remembered of the Daleks, before the Doctor had stopped the war, was watching her mother get killed. She remembered how her mother's face had lit up when she saw Harry from her hiding spot, and had begun to stand up and stretch her arms out towards her. She remembered seeing a Dalek appear, seemingly from nowhere, behind her mother, gun aimed and ready.

Harry didn't even have time to shout a warning before the Dalek fired, and her mother was lit up from the inside. She had shot at the Dalek before her mother had hit the ground.

It had been at that moment that the Doctor appeared, knackered old TARDIS wheezing into existence beside her. It had taken him only a moment to assess the situation, and another to properly register that his wife was dead.

Harry had watched as her father ran over to the body of her mother, picking up her shoulders and gently cradling her to him as he sobbed. Had it been anybody else, they would have simply regenerated and kept fighting; but it was their mother's final life. She had lived for a long time, thirteen very full lives; but it still wasn't fair, to have her last life cut off so abruptly.

John had joined Harry at that moment, slipping his hand into hers as they watched their father grieve. They both had tears in their eyes, but neither let themselves begin to cry. Instead, they bottled up their emotions like always, ready to take it out on the Daleks later.

Quite some time later, the Doctor had risen from his knees, carrying the body of his wife. He had shakily walked towards John and Harry, who silently parted to let him through into his TARDIS. Inside, he had laid her body on the floor and flown away to a safe location. As a family, they had tied their wife and mother to a pyre made of local wood, and set fire to it. They stood, shoulder to shoulder, and watched silently as the flames consumed her.

They had waited until the entire pyre had been reduced to ash before they turned and headed towards the TARDIS. It was then that their father had taken them to Appalappachia and left them there to go and stop the Time War.

It had been nearly a century before Harry had seen either of her remaining family again after her mother's death. He had had a new face and a new TARDIS interior, and did not mention his late wife even once during his time with Harry. She supposed he must have gotten over her, although she did not really believe that he could.

Now, walking through the Dalek ship with John, Harry noticed that she had tears in her eyes once again. She did not let, squeezing her eyes closed for a moment and bottling up her emotions. They could be used on the Daleks, if and when they ever found them.