This is a really exciting project for me because it's not only the first time I've written for Sherlock or Harry Potter, but it's also my first crossover and my first joint fanfiction! The chapters will be written as follows: one from me, one from my friend etc. She does not have fanfiction so all chapters will be published on my account only. Just something to remember: this is not linked to any of my other fanfictions, so Ally does not exist in this AU. There may be sequels to this story, but we're not promising anything because we have a storyline worked out but we don't know how long it will be and what the chapter lengths will be. Enjoy, and please excuse the two obviously different styles of writing!

Sometimes, the Doctor regretted getting involved. When it went wrong, or when people got hurt because of him. It may surprise you to know that sometimes, just sometimes, people got hurt but he didn't regret it. Because it wasn't his fault. Or he made a friend. Or both. One example where both happened was in the war in Afghanistan, when the Doctor just couldn't resist…

Dr John Watson peered out of his tent and surveyed the surroundings. There had been another attack last night, so he had had his work cut out, saving lives, performing risky surgery, the usual day at the office. He wouldn't have done it without the help of the mysterious stranger that appeared like an angel the night before, just before the attack began. He claimed to be a Doctor, and John was in no position to argue, he was just too busy. He somehow trusted this man, even though he was dressed in tweed and a frankly outrageous bow tie just wandering about in the dark. But he guessed he wasn't a Doctor. He was wrong. The mysterious tweed man had saved more lives in an hour than John had saved in a lifetime. Not that John wasn't skilled at his job; he was the best of the best. But there was something about this man that just always knew what to do.

John was from a fairly well-off background: educated at Bart's, trained as an army Doctor and left before his parents could object. He had never got on much with his sister, Harry. She was having trouble, though, with her partner, Clara and drinking, and so John worried about her constantly. For the past few weeks, though, he had had no time to worry. That was all about to change.

John walked round to the side of the tent and saw the Doctor sitting there. His wild quiff was blowing gently in the soft wind. He gazed into the distance, and John stepped back in surprise, as the eyes of the young man were way beyond his years. He was staring at the eyes of an old God. When he spoke it snapped John out of his reverie.

'You OK, Doctor? Busy night last night,' the tweed God uttered.

'Uh, yeah, I guess,' John replied dazedly. 'Hang on; have you been outside all night? Do you even sleep?'

'Yes, I always sit outside at night. Failing that, the Medusa Cascade or the moons of Dotyos make a lovely spot. Sleeping? Nah, sleeping wastes time. There is so much to see, John, so much to observe that has never been observed before. Whole new galaxies, worlds to be discovered, civilisations to save, and a whole lot of running to do, and right in the middle John, it's just you and me, with our whole lives ahead of us. Me, I've people to save, things to do, battles to fight. But you, you've got your own war to win. I'm not just talking about this one we're both in the middle of, but your whole life, your human nature, your instinct. And that is beautiful. Don't lose the battle, John.'

'Uh… Ok,' John couldn't think of anything better to say. 'Do you want some coffee? I can tell one of the boys to put the kettle on.'

'Oh no, I'm quite all right. Actually, I'd better be off. Things tend to go wrong if I stay too long in any place or time. But thanks, John. For everything. And for being there.' He began to stroll of into the desert.

'Wait, Doctor! Where are you going? There's just the desert out there! You'll die! And, we need you, Doctor, you're a life saver. Doctor!?' But the ancient God made of tweed was gone. John hung his head dejectedly, knowing the strange man would die. And John needed his help.

He saw the attack before he heard it. He had often wondered what being caught unprepared would feel like. He had hoped he would go down in dignity, but he found himself rooted to the ground, completely exposing himself. If ever he wished for the sight of a tweed jacket, now would be the time. Or even that ridiculous bow tie. Or…

And with a sharp pain in his shoulder, he keeled over. His last thoughts before he blacked out were that of an ancient God trapped in a young man's body. And those old eyes, staring out into the vast desert.

Three Earth years later, in the TARDIS

'Don't travel alone, Doctor'. River's words kept circling around his broken mind. He needed help more than ever. The pain he felt was beyond unbearable. It was as if one of his hearts had literally been torn from his already wounded chest. His whole body ached with their absence. What hurt the most was that it was his fault that two more beautiful young humans were gone. How many would it take for him to stop? How many deaths before he became like the mind of a Dalek? How many broken hearts, lost lives, lost loves…? How many times would he have to die to be dead? To be gone. To have never existed. He wanted that more than ever, now. His friends kept him alive. And now they were gone. Again. And it was his fault, yet again. It always was. He knew he needed help, just to keep him going, but he had almost lost hope. He couldn't expect River to always be there, their timelines were too complicated. There was only one person he could think of that would help him, one person that he wanted to see… And he didn't even know them that well. He had only known them for one night.

221b, Baker Street

London, England

21:41pm

At the sound of knocking at the door, Sherlock Holmes yelled 'John! JOHN!' When there was no reply, he groaned and moaned and heaved himself reluctantly out of the chair. John had probably gone to Dublin, again. Or something. Sherlock Holmes had always seen himself as a human, merely striving to be a God. And in his mind, he had succeeded. He just couldn't imagine being anyone else. He certainly never thought he'd meet someone who was the complete opposite. Well, the great consulting detective was about to meet his match, and with it get the shock of his life…

'Hello! I'm the Doctor, can I come in? Thank you.' He barged in past Sherlock, leaving him literally lost for words, something that both intrigued and worried him, and he stood in the doorway nervously. He could hear the Doctor from down the hall. 'This is John's flat, isn't it, I mean the TARDIS definitely said it was here, and I always trust my old girl, never let me down she has, and never will, not if I can help it. Ee by by gum.' He stopped, silent, and Sherlock tensed. The Doctor turned, and walked back slowly, until he was right in front of Sherlock. He prodded him, and Sherlock tipped back, and then forward again. 'You're not John. Who are you?'

Sherlock grinned. Now it was his turn. 'Well, let's talk about you shall we, I mean you just barged into my flat without any hope of introduction so let me deduce you and I'll soon suss you out. Sooo, by your dress sense I'd say you were one of John's barmy army friends, probably on leave, before you ask you right sleeve was just screaming holiday, and you don't know me, and I don't know you, so one of his old barmy army friends, and your hair, well you're younger than him but your eyes say otherwise, so, hard time at the front, the way you just strode in here talking nonsense means that you obviously haven't met me yet and don't know what I'm capable of. I just you.'

The Doctor's eyes narrowed for a split second, and Sherlock almost missed it, but not quite. Then his eyes widened into a genuinely mischievous glint and he grinned smugly and held out his hand. 'The Doctor, just the Doctor. And by the way; you're half right.'

Sherlock took it eagerly, it appeared he had met someone with vaguely the similar IQ and intelligence and understanding of the world. 'Sherlock Holmes. Only consulting detective in the world.'

The two geniuses shook hands and grinned at each other, knowing full well that the game was indeed on.

By the time John got home (just from the shops, apparently, not Dublin) the pair were in full flow, arguing to their heart's content. John stopped and dropped the shopping when he saw who Sherlock was debating with. 'Doctor?! Oh, my God what the hell are you doing here? I haven't seen you for three years, and I thought you had died!'

The Doctor grinned. 'Well that's just me, I'm afraid. Unexplained is my middle name,' when John laughed he frowned, and John silenced himself. 'No, literally, the looks I get when I fill in a form…'

Normally this would have just annoyed the hell out of John, and of Sherlock, but Sherlock was already annoyed at having been beaten at an argument for the first time in his life, and John was just too surprised to feel, or even notice, anything else.

The three were sat around the table eating an already cold Chinese takeaway. Neither the Doctor nor John were speaking to Sherlock, he had predicted the fortune cookies and had been proven wrong by the Doctor. John was really beginning to see how similar they were to each other. But he kept it to himself and just grinned to himself and sniggered smugly sometimes, knowing full well that if he even mentioned that he'd entertained the possibility; he knew that World War III might just break out, and he was anxious to watch the football. It suddenly struck him as to how insignificant he appeared to himself to be compared to these two obvious masterminds. He wondered if they even noticed him. He didn't suspect so. He knew (or thought he knew, anyway) that they were far too busy being clever to pay any sort of attention to him. Of course, this was sometimes true of Sherlock, but one thing that John was sure of was that he was like that with everyone at one time or another. What he didn't know was that the Doctor was quite the opposite, and even though he hadn't known him for too long, he was incredibly grateful to John, he believed that he had saved him from complete depression without even knowing it. In John, he had found a true friend. He thought that it was about time that he repaid his debt to him. And the Doctor was about to change John Watson's life forever. For John, Sherlock and even the Doctor himself, nothing would ever be the same again.

Now, it's a little known fact that John Watson is in fact a massive Harry Potter nerd. Always has been. He's never admitted it to anyone, of course, especially not Sherlock. But at the Doctor's unusual request he revealed all… No regrets there.