A/N: Thank you all for the kind reviews! The crossover part begins in this chapter, and the Doctor will finally appear with the fabulous Donna Noble in the last part! (It's a 3-chapter story). Thanks, have a fantastic day or night, and please review! I still do not own the fandoms.

FantasticFoe: Thank you! I try to update something every Friday night. Next Friday will be the last chapter for this story.

ThePro-LifeCatholic: Aw, thanks! Sherlock is ridiculously hard to write. I think I'm kind of using him as a segway toward writing Ninth Doctor fics, since Nine is probably the hardest character I've ever tried. Haha. The Doctor appears in the final chapter of this story, though!

thegirlwhoneverforgot: Awesome, thank you! btw I have a story (or two!) that I want to add to the community you made. Do you want to take a look at them first? Just let me know! (:

Chapter 2

"Come in, come in, have a seat; make yourselves at home, et cetera," Sherlock spouted from memory as he ushered his clients into the flat.

Willie stared inquisitively at them with her wide blue eyes as her uncle held her casually in one arm.

"Thanks, Mr. 'Olmes," the old man replied, shaking his free hand warmly as he stepped inside. The middle-aged woman with him shook hands as well, though in a jerky, nervous manner before taking a seat.

Sherlock watched their movements keenly. The old man affectionately adjusted the couch pillow behind the blond woman's back, but she ignored the gesture, shooting him a look of insolence when he gave it one too many adjusting pats. Clearly, they were father and daughter.

"We didn't know who else to come to, you see," the old man addressed Sherlock after sitting back himself. "We were hopin' you could help us with a missing person? Dy'a do that sort of thing?"

Sherlock wordlessly took a seat with Willie, in his armchair, and settled himself comfortably.

"I told him it wasn't anywhere on your website that you did such things, and that we should've gone to the police, but he insisted that we come here instead, so I do apologize for wasting your time," the woman added, directing a cold stare toward the man beside her.

"Oh, not to worry, Mrs. Noble, I've solved…nineteen…twenty? Missing person cases so far and I will solve yours, provided you give me the right information." Sherlock flashed her his best fake smile.

She started. "Oh, see, that's why I said, too, remember?" she backtracked, nudging the older man, who just rolled his eyes and waited for her to stop. "I said, we never know, so we might as well ask him anyway, just to be—"

"Madam, if you would kindly allow your father to speak," Sherlock sighed, shifting the baby's weight in his arms.

She started again, staring at him as if he were a foreigner. "How did you know he was my dad? We haven't introduced—"

"Never mind that. The case?" he barked impatiently.

"Oh, yes, right t—"

"Your father was speaking?"

The old man was suppressing a grin. "Right!" he began, when the woman finally shut up. "Well, you see, we've got this girl, Mr. Holmes, bright red-headed thing, she's my granddaughter and Sylvia's daughter, and she's the most important thing in the world to us. She ran off with this man, he's called the Doctor, and we try to call her but we haven't heard from her."

"How long ago was this?"

Sylvia started to speak, but with a glance from Sherlock she stopped and the old man continued uninterrupted. "Oh, it was ever since that incident with those little fat people, you know. The ones who got transported up into an alien ship? Wish I'd have seen it," he added, muttering to himself. "It would've been brilliant to see some real aliens, after all these years."

"Ah, yes," Sherlock remembered. "The adipose creatures found walking the streets of London two and a half months ago. I heard of the incident. But why tell me that instead of 'two and a half months'? Did this have something to do with her disappearance?"

"Well, yes…" Sylvia began.

"I didn't even see them," her father added, still remorsefully shaking his head.

"I'm really very worried you see…"

"All them aliens in one night and I didn't even see a one!"

"Mr. Holmes," Sylvia burst out at last, her hands shaking in her lap for worry, "I'm afraid my daughter has been kidnapped—by terrorists!"

Sherlock started laughing, a deep chuckle back in his throat. "No, it wasn't terrorists," he reassured her immediately, as she glared at him. "Why does everyone always think of terrorists?" he added to himself, trying to suppress a smile, which John surely would've scolded him for.

"Oh, don't worry so much, Sweetheart, it's pretty plain that the Doctor's another one of them aliens. You even saw them," the old man kept on. "That's who Donna's with. We're just trying to find out if she's coming back, that's all!"

"Well, it's just—" Sylvia started. Noticing the two men both staring at her. "Well, I don't know, it's just—all these aliens and Donna with them? In the sky? It's impossible," she confessed.

Sherlock smiled slightly. "Once you eliminate the impossible," he said slowly, "whatever is left, however improbable, must be true. Mrs. Noble, given the obvious existence of aliens as you yourself have apparently been a witness, are you certain that your daughter being with an alien is completely, beyond a shadow of a doubt, impossible?"

Sylvia opened her mouth, looking as though she wanted to argue but couldn't quite believe what the venerable detective had just said. She made a little, startled-sounding noise, then turned and elbowed her father. "Dad! Did you hear what he just said!" she hissed, sounding shocked.

Her father simply grinned. "That's exactly what I said! I said I saw them in that little blue box, didn't you believe me?"

"All of this is beside the point," Sylvia exclaimed in a huff. "Mr. Holmes, what I really want to know is, is my daughter safe? Is she happy? Is she—is she still going to come back, and when? She has a life here! She was going to get a job!" she wrung her hands, sounding on the verge of tears.

Sherlock smiled slightly. Willie was asleep now, her tiny head resting against his shoulder. He carefully adjusted his holding position so as not to wake her up. Neither of his guests had said anything about her so far, so he must not have looked too out-of-place with the infant. Perhaps he was getting used to this babysitting practice.

"The answer, Mrs. Noble," he began dramatically, "is this. Your feisty, ginger-haired, travel-addicted, small-necked and in all respects MARVELOUS daughter who likes purple nail polish and Listerine, has a troublesome case of repressed sensitivity, and is slightly prone to temper tantrums," he refused to grin at the open-mouthed expression the mother was now giving him— "is perfectly all right and will be returning home within a few hours, around the time of another major alien disaster, probably one involving clones of some sort... She has never been happier to be away from her nagging mother in all her life and has, frankly, grown a bit more fond of you in her absence, though may I emphasize, a BIT. She has no thoughts of returning as of yet but, as I said before, she is soon to do so anyway and will enjoy her very short stay in your home. Be sure and text me when she arrives. I have a particular interest in meeting this 'Doctor'. Out you go! –I mean, have a wonderful day!"

With that, he carefully lifted the sleeping baby and escorted the father (who was so happy at the news about Donna that he was nearly dancing his way out the door) and his still-open-mouthed daughter (who was making little shocked squeaking noises every couple of seconds) to the stairs, shutting the door behind them.

He sighed in satisfaction as he watched them go. One of the most interesting cases he'd ever had, yet, disappointingly, it was over so quickly.

He made Willie a makeshift bed on the floor with some blankets (the idea again, courtesy of the Internet). He found himself, however, returning to the windows multiple times to check if Mary's car had parked outside the flat. Time and time again, the side street was always empty.

He worked on some experiments (ones that did not involve hazardous chemicals, because Willie must NOT be allowed to breathe those into her tiny, delicate lungs), heated up some leftovers of a dish Mrs. Hudson had made a few days ago, and checked the clock again. It was already 7:06. Willie woke up and he gave her a bottle (he was quite the expert at baby-feeding by now), let her play on the floor with a plastic cup and a soft-edged ladies' pocket mirror he had acquired on an important case, and busied himself checking his emails until Willie fell asleep again at 8:32.

He began to worry as the hands on the clock swung 'round and the street outside began to grow dark. After his experiments were concluded, he proceeded to watch Willie, sleeping in her makeshift bed.

Her face was so unlike his, when he looked in the mirror.

So peaceful, and sweet.

Surely John and Mary would not have left their precious girl with a potential-energy disaster like him, for this long, without a good reason.

Finally, he told himself that worrying would do no good. Perhaps he could treat it like a case.

"The most perplexing cases usually have the simplest explanations," he reminded himself. Mentally, he sorted through the possibilities and looked for simple ideas.

Perhaps John really WAS ill.

That was—that was not even something Sherlock would think about. John couldn't BE ill, at least not seriously. There were hundreds of other possibilities, besides.

There was a social event, perhaps. Maybe one or more of John's less-than-deserving family members had suddenly died and they were attending the funeral. Maybe a so-called 'friend' of high social status was hosting a party, or a fundraising dinner, or being married.

Yes, that was likely.

Sherlock bit his lip and slung his lanky frame over the couch, twirling a cigarette (before remembering that it wouldn't be good for Willie and switching it for another handful of nicotine patches).

He was about to retreat into his mind palace to do some organizing when he recognized a funny smell in the room. His eyes popped open again.

As Mrs. Hudson frequently reminded him, funny smells were usually not good.

Sure enough, there was a cloudy sort of gas coming underneath the door, and more of it seeping in around the cracks in the windows!

Sherlock leapt to his feet. Someone was trying to gas them out.

"Moriarty? A cohort of Magnusson's? Black Lotus?"

It didn't matter. Another sniff told him the exact potency of the gas, and that once it filled the flat, they would have a maximum of eleven minutes before losing consciousness. Willie would probably have less.

He also noticed it smelled very similar to—car exhaust.

"Carbon monoxide, hydrocarbon, nitrogen oxide…where's it coming from and why is there so much of it?"

He ran to where Willie was sleeping on the floor. She blinked sleepily and started crying as he picked her up, along with his cracked phone, and ran to the back bedroom where he'd stored some surgical masks for working with vaporizing chemicals.

Strapping one over Willie's face (it completely covered her eyes and forehead) and one over his own nose and mouth, he ran downstairs.

"Mrs. Hudson?" he shouted, trying not to breathe in more of the gas.

When there was no answer, Sherlock mentally kicked himself. Of course she was still out with her—whoever it was she was out with. He dialed John. It rang and rang, but no answer. Then Mary, but the same result—only voicemail, which he didn't wait long enough to listen to.

His mind quickly catalogued recent events—the call that had come that afternoon, Mary dropping off Willie, the adventure with the stove, the clients. What had he told them? There would be a major alien disaster? Had he just assumed said disaster wouldn't affect them at Baker Street?

"Stupid, stupid, stupid!" He wouldn't be making that mistake again.

He ran to the window. Civilians were running, coughing and screaming, away from their cars and into the houses, holding handkerchiefs and bags over their faces to avoid breathing in the gas. The fumes appeared to be coming from the ATMOS systems underneath the vehicles parked outside—hmm, interesting.

His final bit of research consisted of checking the news report on his phone. "WORLDWIDE GAS FLOOD CHOKES INDUSTRIAL CITIES. WILL THIS BE THE END OF THE HUMAN RACE?"

"None of my concern", was his first thought, as he rolled his eyes. Negotiations and political problems regarding aliens weren't exactly his forte. Ask him to locate an alien being or lone agent? Another matter entirely!

Sherlock shook his head. He couldn't exactly save the entire world. But he could save himself and Willie, even if he only had minutes to do it.

Baby in arm, he sprinted up the stairs three at a time, slamming the door shut behind him. The fumes had already reached higher than the flat, so there would be no benefit in getting up onto the roof.

He hastily stuffed Mrs. Hudson's holiday table runner under the door, blocking the fumes partially, and plopped Willie down in a closet with more stuffing under the door. That would at least give her a little more time.

He dramatically swiped out the whole table of experiments, letting the materials crash to the floor unnoticed.

"Gas sample."

He grabbed his supercooling solution in its convenient aerosol can. It had been a gift from Molly and treated with a significantly greater level of affection.

If he'd been more sentimental, he might've vowed to treat her more nicely, if he ever got out of this alive.

Quickly, he sprayed a thick cloud, condensing a few drops of gas into a liquid onto a spot plate.

"Six minutes left."

One solution in each drop of condensed gas…His eyelids were already beginning to droop…He had a suspicion the poison would be neutralized with cuprous chloride and one other chemical. There were many solutions that could possibly work, but ammonia seemed like the best bet.
With those two chemicals, he had the carbon monoxide neutralized, however, the gas wasn't straight car exhaust. There were a few other chemicals he wasn't quite sure how to neutralize.

"Five minutes."

He spotted another drop with the remaining ingredients to absorb the hydrocarbons. What else was it that he needed? He desperately hoped it wasn't some sort of alien compound. In all likelihood no chemical on Earth could absorb a foreign chemical.

"Four minutes."

"Think, think, think, think, THINK!" He gagged on the fumes as they started to grow more concentrated in the room. His vision grew spotty and for a second he just wanted to collapse on the floor and sleeeeeep.

"Three."

Whether it was a hallucination or just a memory, he didn't know, but he thought he saw Willie smiling at him, gurgling happily like she had earlier that day.

Sherlock had never been given charge of her, although he'd always loved her, since the day he shot Magnussen to ensure she would have a childhood, and that other day when he'd held her beside Mary's bed, fresh from the womb.

Her first time in his charge would be the last time.

She was dying.

John's baby couldn't DIE.

It wasn't even an option.

John's baby WOULDN'T die, not when Sherlock Holmes was around to save her!

"Two."

Out of the fogginess of his blackening brain, he had an idea.

The additional chemical wasn't being absorbed by the current ingredients, but perhaps it would be if its current state was altered. He'd condensed the gas so he could work with it at the table, but what if he already had the solution right? What if the reason it wasn't absorbing the condensed gas was because it wasn't actually in its gas state?

Willie had refused her bottle earlier. It had been too cold.

The gas was refusing the neutralizers because they were too cold. The additional elements, surely, were only altered versions of Earth's, only adjusted to different climates.

He had to heat up the neutralizer and feed it to the gas, just like a warm bottle.

Ugh. "So domestic."

"One."

He stumbled over to the stove and turned all the burners up on full heat. Rapidly, he combined all of his final solution, hoping some of the labels were right because he was nearly unconscious and finding most of the ingredients by location only as his vision grew spotted.

He swirled it all around in the biggest Erlenmeyer flask he owned, waved his hand over the stove to make sure it was hot enough, and poured a stream of liquid onto the hot surface.

Zero.

A huge cloud of steam billowed up, and only falling to the floor unconscious saved him from a serious burn as it rose and filled the room.