AN: I made my way quite recently all the way through Nine up to Ten, and just started Eleven. Doctor Who has drawn me right the way in, and I just had a random little idea for a cross over. However, there are lots of details I haven't worked out yet, so updates may take a while if I manage to get through this story. The last fic I started on a whim kind of fizzled out of me, but I thought I'd give it another go. Will try to preplan a bit more of this one before posting any more, but hope you enjoy it anyway!

I chose Ten and Donna because they are my favourite.

A journey in the TARDIS was rarely a smooth one; the ship designed for six pilots had for a long time only been directed by one, who was currently running circuits around the control panel as he tried to steady an unusual amount of turbulence.

''I'll smudge my eyeliner in a minute!'' Donna scowled, holding her travel mirror steady on her lap, sticking out a boot so that her make up bag couldn't slide away as the TARDIS tipped.

The Doctor lunged for a set of buttons to his right, perplexed as the ship continued to shake in the vortex.

''Come on, stop being so naughty-'' He muttered to the control panel, glasses glowing blue from the lights that winked at him disobediently. It wasn't playing nice. But it couldn't help it. Something was pulling the TARDIS off their planned route, something small but significant, but he couldn't manage to trace it with all the rocking around. A thin hand flew up to grab at short brown hair, already ruffled in its usual style.

''We're going to have to make a detour, I can't pull us out of this- we're caught in something, like a drain, it's pulling us down-''

''A detour where? What's pulling us down?''

Donna watched her makeup bag shoot across the TARDIS floor. She let it go, mirror still in hand as she stumbled across to the control panel. None of it made any sense to her; she wondered sometimes how much of it actually made sense to The Doctor…

''Oof, there we go, we're out of the slipstream now! I should at least be able to land us…''

''Land us where?''

Despite having no clue what had gripped the TARDIS, The Doctor's eyes were alight, grinning across at his red headed companion.

''We're about to find out!''

''All of time and space and your default is London…'' Donna muttered, retrieving the various contents of her bag from across the TARDIS floor. She shoved them back into the bag along with the mirror and zipped it with relish, tossing it to rest on top of the control panel.

''Nothing wrong with London. Especialllllly- ooh, Baker Street...''

''Yeah, right. Amazing…''

''Something yanked us in this direction, I just need to work out why…trying to get a scan running.''

''Right. Well while you get things sorted in here, I'm going to grab a coffee.''

''Ooh, Latte for me,'' The Doctor responded, focused on the TARDIS controls. He could feel that look, though, and as he raised his eyes to Donna again he saw the raised eyebrow and the ghost of a smirk on her lips. ''What? What?''

With a sigh, Donna Noble stepped out of the TARDIS. The air was unmistakeably London,though why she couldn't exactly explain in words. It just was, and she knew it all too well, shrugging her jacket around her a little more against a chilly breeze. It was wet out, too. Of course it bloody was. She took a few steps forward towards the main street, casting her eyes up and down the road for the nearest coffee place.

Something niggled in her memory as she did so. She frowned, looking from one end of the street to the other, and back again, trying to pinpoint what it was.

''Wasn't there some museum on this road or something?''

''Actually make it an espresso, think I need a jolt to figure this out. Looks like there was some sort of glitch as we were passing…'' The Doctor called, still fiddling with the controls.

''I swear there was…Doctor, the Sherlock Holmes museum's gone.''

The Doctor suddenly appeared, sticking his head out from around the TARDIS door and looking supremely disappointed.

''Awww, no, I liked it in there, the people all dressed up in character and the old floorboards- have they closed it down?''

''No. It's just….gone. Like it weren't even there…''

She indicated the space, which presented nothing now but a small shop dedicated to the Beatles and a slightly-better-than-grim café and sandwich bar called Speedy's.

''Donna.''

She looked in the direction that the Doctor was staring over his glasses, her gaze settling on the black door of a house. The knocker was askew, the numbers above reading 221B.

''Yeah, so? This is Baker Street, that's the address Sherlock Holmes lived in.''

''Yes, but there is no 221B…there never has been. It was always fictional and now…here it is. And the museum's gone, just gone…oh.''

''What?''

''Oh!''

''What, spaceman?'' Donna knew this game could go on forever if she didn't snap him out of it sharply.

''That's it,'' The Doctor said, his grin at full elasticity, ''another universe, this is another universe, separate from yours and almost the same except that here, 221B is real, and someone lives there.''

''What, d'you mean- no. No, shut up, Sherlock Holmes isn't real, he was made up like a hundred years ago by what's his face.''

''Arthur Conan Doyle, yes. But there was a glitch in time. Something's happened in this universe that wasn't meant to, and the TARDIS was drawn to it…this is where the source is.''

''How can Sherlock Holmes exist in another universe? How?''

''A story has a life of its own- when you create a story, you are literally creating life, creating something out of nothing but words and imagination, and that must create universes where those things are reality…it's just a different reality, Donna, do you see? Every decision you make can spawn a dozen different universes, all focused on the other decisions you could have made, but didn't.''

''Are you telling me that every time someone writes something, it makes another universe?''

''Quite possibly. Well, pretty likely. Welll- yes.''

''So, Sherlock Holmes is up there, right now.''

''Yes.''

''Every story?''

''Yup.''

''So somewhere out there in another universe, there's a real Jack Sparrow that looks like Johnny Depp?''

''Umm-''

''Oh my God!''

''Well, he might not look the same, but essentially…oi! Anyway, we need to work out what happened here, what caused the glitch. Donna-'' The Doctor was beaming at her, before looking across the road to the glossy black door, ''let's pay a visit to Sherlock Holmes.''