(I've had this story/crossover in my mind for a while now. It was meant to be a short story but I can't do short [even though I've cut a lot out] so apologies. As with all my work [you if ever come across them on my twitter] I don't tend to check paragraphs/punctuation after I've finished as I find it too tedious so apologies for errors there.

Enjoy)

"That's a skull."

"A friend of mine. When I say friend..."

Alone in his flat, Sherlock stood before and then sighed at the skull on his mantle piece and the memories of when John Watson had first seen it. The day he'd received that skull was the clearest memory he had; far clearer than any other memory tucked away in his mind palace.

That memory was also his biggest secret. It baffled him and Sherlock Holmes did not get baffled. It was impossible. Sherlock wrinkled his nose in disgust at that thought. Impossible. That was supposed to be an unknown word to him. There was supposed no such thing as impossible but how the skull came into his possession was most definitely impossible. Sherlock lightly brushed the skull with the tips of his fingers as if to check it was physically there. It was.

'Impossible' he scoffed quietly.

The case of the impossible skull. The case of the impossible skull that the great Consulting detective could not solve; that was why it was his biggest secret. Sherlock sharply turned his back on the skull and walked away to retrieve some notes from the other side of the room. He sank down into his chair, deliberately positioning himself facing away from the mantle whilst trying to concentrate on a case he was currently working on. For good measure he took in his notes and then shut his eyes.

Wife. Dead. Husband. Innocent. Guilty. Innocent. Murder weapon? Wound? Blood. Head. Fracture. Skull. Skull. Skull.

Oh.

Sherlock's eyes snapped open and that thought and he turned to glare violently at the skull and threw his notes across the room to indicate it had won his attention. He shut his eyes once more, pulled the memory from his mind palace and let it consume him.

He was ten years old. It was the middle of the summer holidays and Mycroft had just been engaging in his favourite activity: annoying Sherlock. He'd gone into the garden to get away from him and to check on his worm experiment. Sherlock could remember everything. The bright sun, his worm experiment flourishing, the sound of Mycroft having a tantrum inside the house, the sound of... the sound of...He never could explain the sound. All he knew is that it came from the very bottom of the garden where the trees and overgrowth consumed the bright sun and that it had caught his curiosity.

He glanced back at the house to see if anybody else had heard it. No. He apprehensively walked down the garden and stopped at the start of the trees and overgrowth. He strained his ears but could hear nothing out of the ordinary now. He couldn't see anything unusual either but the garden carried on for at least another 25 metres before a battered fence marked the territory between domestic overgrowth and wild, dense woodland. He pushed his way through the overgrowth almost to the very end of the garden before looking around. Trees. Bushes. Brambles. More trees. Sunlight obstructed. Darkness ahead. Darkness behind. Nothing unusual. What had caused that noise then? He stood for a few moments wondering if he'd imagined it. [You don't even know what it was. How could you imagine it then?]

Suddenly a there was a snap of a twig [Close]. Sherlock stood statue still, listening hard [Footsteps. Human not animal. Male? Definitely male]. A man emerged from between two trees a few metres from Sherlock. Sherlock took in his strange dress sense, especially the tweed jacket and red bow tie. His clothes were a random mix and gave no indication of where he'd come from before the bottom of the garden. The man grinned broadly when he saw Sherlock staring and then promptly tripped over a tree root.

"Trees! Wood!" he exclaimed as he picked himself up. [English. Mixed region?] "The trouble I have with it!"

Sherlock continued silently gazing at this strange man trying desperately to deduce something concrete about him. The man grinned again as he stood in front of Sherlock.

"Hello!"

He took Sherlock's hand and shook it for an uncomfortably long time until Sherlock pulled his hand away.

"Oh, I'm sorry! I can never get the hang of greetings! Shaking hands, kissing...other things. You're only supposed to shake for a few seconds, is that correct?" He didn't wait for an answer. "Yes. Must remember that!"

"Who are you?" Sherlock asked. He couldn't help himself, curiosity had won.

"Yes! Again, sorry! Rude, Doctor, rude. I'm the Doctor. Hello!"

Another handshake. This time it only lasted several seconds and the man looked pleased with himself at getting it right this time.

"The Doctor?" Sherlock frowned slightly. "That's a title, not a name"

"It's a title and my name. That's what people call me. That's what I call me. The Doctor! I had a name badge once. It said my name on it. Lost it now but you can remember my name, can't you?!"

"Okay, the Doctor" Sherlock humoured him "Why are you in my garden?"

"I'm here to see you, Sherlock"

"How do you know my name?" he asked suspiciously.

"Oh, Sherlock Holmes, the great Sherlock Holmes, you're brilliant! Your mind...Genius!" The Doctor frowned at him slightly and then looked him up and down.

"But you should have worked it out by now. Maybe I'm too early".

He held up his hand to Sherlock's head and then to his own chest as if to measure Sherlock's height.

"Yep, too early. Stupid, Doctor! Never get it right! Oh well, this will have to do otherwise it'll get wibbly wobbly."

Sherlock was very curious now but he couldn't work out why. Maybe it was the oddness of the man or the fact that he wouldn't give his proper name or the fact Sherlock couldn't really deduce anything about him, or maybe it was a mix of the three, but curiosity was keeping Sherlock in the small clearing in dense overgrowth with the odd man.

"How did you get here?"

"I sort of landed over there" the man waved his finger around before pointing vaguely towards the very end of the garden "and then I walked around, got a bit lost, then saw you"

"What do you mean 'landed'? Your shoes show you've only walked through mud and leaves specifically from the ground around here and the distribution of mud shows you've only walked, that is, you haven't jumped from any height so you can't have landed in that sense. I didn't hear any planes or helicopters and there's no way they could land in here anyway so 'landed' in that sense is out too"

The Doctor laughed and clapped excitedly.

"Oh, Sherlock, you! As a matter of fact, I did land. In a ship".

Sherlock didn't even dignify the obvious lie with a look around for a ship. The Doctor laughed again. Sherlock watched as he adjusted his bow tie and fidgeted like an excited child waiting for something.

"I'd better be getting back to my worm experiment" Sherlock announced after a few moments of fidgety silence.

"Worm experiment? Oh, can I see?! I love worms! Went to the planet Lumbricidae once. Giant things but not very clever. Almost become king of them...That was a good night..."

The man carried on chatting about a worm planet as Sherlock watched him closely. This man was quite improbable and Sherlock was not a fan of that. He made to leave when the man suddenly stopped rambling on noticing Sherlock moving.

"Oh, I'm sorry. I was rambling, wasn't I? That tends to happen...a lot. Isn't that what you humans do often? Or some do...The ones I meet. Ramble! Funny word. Did you know-" He stopped again on seeing Sherlock's stare.

"What do you mean 'you humans'?" he asked taking a step closer. This man, this doctor was clearly mad but there was something about him that intrigued Sherlock. He was a puzzle that needed solving.

"Well, like I said, I'm the Doctor and I'm a time lord. The last of the time lords actually. I'm from the planet Gallifrey in the Constellation of Kasterborous. It's a beautiful planet... was a beautiful planet. It's gone now." The Doctor was staring in to the distance, reminiscing.

"There's no such thing. Besides you look human"

The Doctor snapped out of it, looking Sherlock in the eyes [Shining. About to cry.]

"No, you look time lord. We came first! And listen..." he pulled a stethoscope out of his jacket pocket "two hearts"

He handed the stethoscope to Sherlock and moved it across his chest for him to hear. There were definitely two heart beats. Sherlock took the stethoscope from the Doctor and put it to his own chest. One heart beating. Racing. [Excited? Nervous?] He shook his head and handed it back to the Doctor.

"But that's impossible. Impossible" he spat and then deeply frowned.

"Sherlock Holmes. You are wonderful, clever, and so brilliant! But the things I've seen and the places I've been. Nothing is impossible as you think. It's true! Oh, your mind is so beautiful!" He kissed his fingers and placed them on Sherlock's head "But it's so closed at the moment. Children your age should have endless, imagination. Impossible imaginations. Yours is cold, hard facts. You need to see something impossible"

Sherlock knew he should turn around, check on his worms, and go inside. He was sure he was dreaming. Yes, dreaming. Because a man with two hearts, a time traveler, somebody who seemed to know his future, could only be possible in his dreams.

Sherlock opened his eyes and the vivid memory dissolved. He could almost scream at his younger self not to follow that man with the two hearts and the strange clothes. He cursed out loud to the skull and gave it his dirtiest look. This was why he kept this memory locked away in his mind palace. This was why he hardly ever relived it. It made him feel emotions. Human emotions. Now that was an alien concept to him. Confusion and anger coursed through him. Sherlock snarled at the skull. What good were emotions when they didn't offer any benefit or change anything? He shut his eyes again.

Back in the clearing in the depths of the overgrowth, the sun was lower in the sky so the light had shifted. It was casting almost eerie shadows if Sherlock believed in such a concept, which he didn't. Sherlock deduced that the Doctor had noticed the change in light too. The Doctor licked his finger, held it up in the air, and exclaimed the light was shifting. Not his finest deduction.

"Blimey, time's running out, Sherlock. Now you have to make a crucial choice which will change your whole life. And I'm sorry it's so early on in your life, Sherlock, really I'm so sorry but I got it a bit wrong and I can't come back later now so it's up to you. And it's entirely up to you as this is not a fixed point in time; it's a chance to change everything. What happens here, right now, will shape your entire future. It's all down to the choice you make right now. All a bit rushed for me to explain, I admit; I need to stop rambling on these visits."

"You're a...time traveler. Time lord, not human, measuring my height, you arrived too early in my life...The two hearts...Impossible" Sherlock shook his head in disbelief and confusion.

The Doctor smiled sadly at Sherlock. A ten year old boy with his whole life ahead of him. His name would bring fear and hope to so many people in the future. The things he'd do in his life with that brilliant mind of his. A genius! Brilliant! Fantastic! But the choices he'd have to make. Nobody should have to make those choices. The people who would enter Sherlock's life and stay there would be the luckiest people ever. But the ones he cared about the most would also fall the hardest and break from the problem he'd eventually face. They'd be broken forever from meeting Sherlock but be held together by him at the same time. Oh, they were so alike, the Doctor and Sherlock Holmes, and it pained him so much to look the ten year old boy in the eyes as he knew the responsibility and the burden it all brought. But it was wonderful, so wonderful at the same time. The best. The days he would have.

"Yes, I am a time traveler and you're wrong about me not having a ship. It's a space ship! Now, you can choose to come with me over...well I'll find my ship eventually and it will change how you see everything. Open your mind. I told you I came to see you because I have a gift for you, that's the purpose of me being here. But, you can also choose to walk away now. Go back to your worm experiment and then go inside and forget this. That is, if you don't believe any of this and think it's just an impossible dream about a mad man. Forget this."

Sherlock looked around him. It was getting dark fast and his parents would be calling him for dinner soon. If he was going to make a choice it would have to be now. The present day Sherlock- the one reliving it now- still couldn't work out why he'd chosen to go with the mad man. Perhaps he wasn't thinking rationally then or maybe it was only true curiosity a child could experience. He'd agreed to go with him whatever it was.

The Doctor had grinned widely when Sherlock told him he'd go with him and see his...space ship. Both child and adult Sherlock winced at that term. The Doctor pulled out a device from his pocket "A sonic screwdriver" he explained as he pressed a button so it made a whirring noise. He waved it around in the air before nodding and striding off.

"Well come on!" he called back to Sherlock "and no wandering off!"

They walked towards the very back of the garden, the light continuing to fade. Sherlock shivered slightly, his light summer t-shirt and trousers failing to keep out the evening chill. At least that's what Sherlock tried to fool his memory into thinking each time he relived this. He'd seen it, he'd seen the space ship and it had made him shiver. Why? It was stood by the fence in a smaller clearing than they had just come from. A 1960s police box, not a stereotypical child's view of a spaceship. Sherlock eyed the smallness of it. Small. Very small. [Disappointing?]

The Doctor ran up to it, patted it, and happily laughed. He turned to face Sherlock who was stood a few steps away.

"Isn't she beautiful?! She's called a TARDIS." He held up a key.

"Time And Relative Dimensions in Space. Travels in time and space. Any where in time and space". The key in the lock. [Heart beating faster. Why? Breathing more rapid. Excitement? Fear?]

"Now, what you're about to see is very special and not many people get to see it. It's also going to open your mind to the impossible and hopefully change you in the best way possible!" The Doctor turned the key.

[Don't go in. There's still time to turn back. Just go! The mad man with the police box is less of a mystery than if you go in there. Turn back!]

Another shiver as the Doctor pushed the door open, smiled at Sherlock, and gestured for him to step inside.

"Im-poss-ible" Sherlock breathed as he stepped across the threshold. Big. Very big. Bigger on the inside. He backed out, past the Doctor who was still smiling and holding the door open for him, and walked round the outside. He touched the exterior tentatively. It was real. Everything was real. It had to be a dream then. After several walks around the box he walked back into the TARDIS and found the Doctor rooting through a large suitcase. He looked up as Sherlock approached.

"You're not dreaming. Though that doesn't really prove you're not dreaming because dream me would say that. I would pinch you but you can feel pain in dreams too." He pulled out a mirror from the suitcase. "Dreams can make it very hard to distinguish between reality and fantasy and that's especially the case when you're in the TARDIS. Both feel equally real." He held up the mirror to Sherlock's face "But, fantasy can never fully reflect reality if you know where to look. This is important. Look at yourself, Sherlock. Notice how it's just you. Fantasy distorts reality and in most cases, that is in dreams, it distorts faces the most. Your face is perfectly normal. Perfectly real. This is all real life"

Sherlock watched his face in the mirror. It was perfectly normal apart from his pupils were dilated. [Excitement]

"I don't...I don't understand" he finally muttered as the Doctor put the mirror back in the suitcase.

"Good" the Doctor smiled at him "That means you've opened your mind to the impossible". He looked at his watch. "I'm sorry I haven't got time to explain it all"

"I thought this was a time machine"

"It is but I can't...I can't travel with you. I lied to you, I'm sorry. This is a fixed point in time where it has to be this way and always this way. I had to get you to come here by thinking you had a choice. I can't change anything. I arrived too early in your time line but that doesn't change the fact I can't travel with you. I can only give you your gift and then leave you. That's just how it is, how it will always be, and I'm so sorry. What happens here must always happen here or I could destroy your entire future and I can't take that responsibility. This had to happen like this. I'm sorry I lied."

"Then why are you showing me all of this?!" Sherlock asked "If I can't travel with you or you can't explain properly then what's the point?"

The Doctor patted him on the shoulder before going back to root in the suitcase. Sherlock looked around him and stood there taking in where he was [Heart rate now normal. Accepting the impossible?]. The Doctor pulled a white box out of the suitcase and handed it to Sherlock.

"This is the point. Open it carefully"

Sherlock slowly prized the lid open. A skull. He frowned and gently lifted it out.

"Not just any skull, it's-" His watch beeped. "Ah, out of time". He pulled a thin white envelope from his pocket [Pockets bigger on the inside too?] and indicated for Sherlock to put the skull back in the box.

"Put the skull somewhere nice and safe and open the letter when you get a proper chance to take it in fully. Guard it with your life because I can't come back if you lose it. Do you understand?"

He nodded as the Doctor placed the letter on top of the box and ushered Sherlock out of the TARDIS.

"It was a pleasure meeting you, Sherlock Holmes. I hope you've learnt something from our brief encounter and I hope you enjoy your gift! I'm sorry this has all been so rushed. I'm not good at the fixed point moments, but you believed in the impossible and I gave you your gift." He went to shut the door as Sherlock stepped outside but poked his head out at the last minute. "You might want to stick around for a few seconds. Good bye, the brilliant Sherlock Holmes"

Sherlock mumbled bye as the Doctor closed the door. He carefully held the box in one hand whilst he neatly folded the envelope up and put it in his trouser pocket. It was almost dark now and almost certainly time for dinner. A strange noise suddenly emitted from the TARDIS, the same noise he heard before all this had started. The wind and noise picked up and the TARDIS started fading from view. Sherlock clutched the box tighter as he looked on in astonishment. The TARDIS had vanished in front of his eyes. He stood in astonishment for a few seconds longer before making his way back through the trees and overgrowth, back through the garden, straight past his worm experiment, and into the house.

His Father called from the kitchen that dinner would be ready in ten minutes. Sherlock quickly made his way upstairs to his bedroom and thankfully avoided seeing Mycroft who would be asking about the mud on his shoes and the white box. He changed his clothes, hiding them, his shoes, the box, and most importantly the letter in his wardrobe where he'd look at it after dinner.

Mycroft eyed him as he sat down at the table for dinner but nothing else was said even when he gave a few worded response to his Mother's question about his worm experiment. There was no evidence that Sherlock had been up to anything unusual and Mycroft wouldn't believe him even if Sherlock told him what he'd been doing. Sherlock ate calmly and behaved as normal as possible but his mind was racing with everything that had happened. He couldn't wait to get back to his bedroom. Mundane conversation and the sound of eating filled the room until everybody was excused.

Mycroft went upstairs to his room to finish learning some foreign language like he always did after dinner. Sherlock followed slowly and headed towards his room. [Nothing out of the ordinary for Mycroft to be suspicious about. All normal. All real]

Sherlock shut his bedroom door and opened his wardrobe. He took the box and the envelope and placed them on his bed. His heart was racing again and his hands trembled slightly as he opened the letter and began to read...

Sherlock opened his eyes again. The skull was the first thing he saw but he felt no anger towards it now. He went over to his locked desk and got the letter out that he'd received all those years ago. It was creased and slightly faded now but Sherlock knew it off by heart anyway. He walked back over to the mantle and looked at himself in the mirror. Normal. No distortions. Real. Not a dream. He began to read the letter out loud to the skull.

"Sherlock,

I hope you enjoyed meeting me as much as I enjoyed meeting you. I am sorry our visit was so brief but I hope you understand why. Our impossible meeting gave me the chance to give you this skull and I hope you take good care of it until you realise properly whose skull it is. I know you'll keep it safe once you do realise.

You're going to meet a man one day. You may have a long wait but he's not just any man, he's the best man you will ever meet. His name is John Watson. You'll deduce it all eventually so I don't need to tell you any more and I don't think I can anyway. Fixed point in time, remember? Wibbly wobbly timey wimey"

Sherlock stopped reading to scoff at the last part.

"All I can say is that John will change your life forever and I went to a great deal of trouble to get his skull and to give it to you. You won't understand fully why I've given it you now or why I've given it you at all but I have. Leave it at that. Take care of him, the living version and the remains of him, and know he'll always be with you. Always. Remember that.

I hope you're putting your wonderful mind to good use and sometimes remember the day when you met a mad man with a box. The case of the impossible skull. Don't let it annoy you too much as there will always be puzzles that are best left unsolved. The important thing is that for a short amount of time you opened your mind to the impossible and embraced it.

All the best to you and to John,

The Doctor."

Sherlock lightly stroked the skull. John Hamish Watson. He was out somewhere at the moment, Sherlock wasn't listening when John had told him where. He was also sat on the mantle in his living room right now though. Sherlock smiled at the skull at the thought of explaining all this to John.

"Impossible" he breathed and smiled. The Doctor was right, there was no point in it driving him mad but that day would always remain a secret. His biggest secret.

"That's a skull."

"A friend of mine. When I say friend..."

Not just any friend. His only friend. His best friend. The best and kindest man he'd ever known who had changed his whole life and would forever be with him.