A timey-wimey Wholock drabble that came to me last night. There's not much to it but it was fun to do.

The doorbell rang. Sherlock was hunched over his microscope, peering intently at some specimen. John was sat drinking his morning tea, paper folded out over his knees. On hearing the doorbell he looked up from the paper, and turned round, looking at Sherlock. He hadn't moved an inch. John sighed. "I suppose I'd better get that, had I?" he said, getting up and putting the paper on the table.
Minutes later, he re-emerged with a visitor in tow. "Sherlock?" he said warily, knowing that he wouldn't want to be distracted from his work.
"Busy," Sherlock replied quietly.
"Sherlock, we've got a guest."
"I'm busy John," he replied, rising in volume.
"You'll want to see them," John replied. "Just for a minute."
"No I won't," he replied in a sing-song voice.
"Look, I'm sorry," John said, turning back to their visitor. "He's quite awkwa…"
"I'll handle this Sweetie," she said, smiling at John before walking straight past him to see Sherlock.
"Mr Holmes," she said to Sherlock. "I've heard all about you. I wondered if you might help me with something."
Sherlock had stopped to look at his visitor. She noticed his shrewd, calculating eyes travelling over her body but she knew he wasn't admiring the view.
"Who are you?" he asked with his eyes finally meeting hers, his analysis apparently complete.
"Dr River Song, archaeologist."
"Oh a Doctor, missed that one, John's a Doctor, why don't you have a word with him?" he said, smiling almost sarcastically before returning to his microscope.
"I'm not that sort of Doctor. Besides, I want you," she said smiling mischievously. John frowned. Was she flirting, with Sherlock? He laughed slightly. She wouldn't get very far.
"Oh?" Sherlock said, looking up again, a smirk tugging at the corners of his lips. John frowned again. He wasn't playing along was he? But Sherlock didn't do that. He of all people knew that.
"I'm looking for somebody and you were highly recommended," River said, her eyebrows raised.
"Who by?" Sherlock replied.
"Miss Irene Adler," River said, smirking as she said it.
"Ah," Sherlock replied, realisation dawning across his face. John wasn't expecting that. But it made sense. There was something about her presence that seemed to draw you in. He wasn't sure whether it was the hair, the heels or just her, but she seemed very headstrong and feisty.
"So. Who are you looking for?" Sherlock asked, his focus partly on her but also partly on his specimen.
"The Doctor."
"What sort of doctor is this Doctor then?" John asked. Both turned around and looked at John as if they hadn't realised he was still in the room.
"Well, he's not your sort of Doctor," River replied. "And he's not my sort of Doctor either."
"Makes sense," John said, rolling his eyes.
"It makes perfect sense Sweetie," she replied before turning her attention back to Sherlock.
John went to sit back down and continued reading his paper. He wasn't sure if he liked this woman or not. What sort of name was River anyway?
"Any idea might have happened to him?" Sherlock asked.
"Well, he seems to have disappeared off the face of the Earth. Quite literally," she replied with a knowing smile.
"Does he do this often?" Sherlock replied.
"Oh, all the time," River said, laughing.
"Why are you here then?" John asked.
"Because this time it's different. Nobody knows where he is."
"And what makes you think we will?" Sherlock replied.
"Well you're a Detective aren't you"?
"Yes."
"Well then. You're my best bet," River replied.
"Okay. When did you last see him?"
"Oh, let me check." She took out a blue journal and flicked through the slightly yellowed pages. "Ahh, here we are. 2045 it was. Remember it like it was yesterday."
John looked up. Had he heard that right? "2045?" he asked, looking over the back of his armchair. "Meaning, 33 years from now?"
"Of course Sweetie," she replied.
"How on earth is that possible?" he asked. "You're not seriously taking this are you Sherlock?"
"Well, it just got a whole lot more interesting," Sherlock replied.
"It's not even… it can't have happened!" John said. Was he the only one who hadn't gone completely mental? "You said yourself, once you've eliminated the impossible, whatever remains, however improbable, must be the truth. When we eliminate the impossible here, nothing remains!"
"Oh John," Sherlock replied. "Have you never heard of the Time Lords?"
River looked impressed.
"I'm right, aren't I?" Sherlock said to her.
"Yes, you're exactly right Sweetie."
"Time Lords?" John replied, still as bemused as before.
"Yes, Time Lords, an ancient extra-terrestrial race from the planet Gallifrey that can travel through time. They've got two hearts, live for an extremely long amount of time, hundreds of years even, and have the ability to regenerate, that is, transforming their bodies into a new physical form, whilst retaining their memories from their previous body."
"Sorry, did I just hear what I think I did?"
"Yes, every word Sweetie," River replied. "Oh and regeneration – loads of fun," she said, winking.
"You?" Sherlock asked, surprised.
"Sort of. It's a long story," she replied.
"Anyway, I really must dash. Are you going to do this or not?"
"Of course," Sherlock replied.
"You're mad" John said.
"Well you're coming with me," Sherlock said.
"No, I'm not. I thought the mystery of that gigantic hound was adventurous enough, let alone a disappearing time-traveller."
"Couple of things you should know first," River interjected. "Look out for a blue box. He'll never be far from it…"
"Hang on, what sort of blue box? Just a blue box?" John asked, clearly sceptical.
"No, a police box."
"They don't have blue police boxes anymore."
"He does," she replied. "If you hear an annoying buzzing, it's probably his Sonic Screwdriver and he likes wearing ridiculous hats. He's always wearing bow ties too, no matter how many times I tell him they're awful. That enough to go on?" she said, smiling.
"We'll try. Won't we John?" Sherlock said forcefully, reminding him that he had no choice in the matter.
"Maybe," John replied, returning to his paper which he still hadn't managed to get through due to the morning's interruptions.
"Nice to meet you Dr. Song," Sherlock said, holding out his hand.
"Likewise Sweetie," River replied, smirking. She took his hand and they shook, before she left as quickly as she had come.
"Please tell me you were joking?" John said, exasperated when River had gone.
"Of course not. You think I'd miss out on the chance to meet a living, breathing Time Lord? Never," he said, smiling for the first time that morning. He abandoned his specimen and grabbed his coat and scarf. "Come on John, we've got a case to solve."
John watched as Sherlock rushed down the stairs. Resignedly, he picked up his jacket and took one last sip from his tea. "Coming," he called after Sherlock.