(Author's Note: The title refers to Tartini's Sonata in G Minor, a tremendously difficult violin piece readily available on Youtube. Spoilers for "The Reichenbach Fall," the ending episode of Season 2 of Sherlock. This takes place midway through that episode, after archvillain Jim Moriarty has shown up at Sherlock's for a cup of tea and some "light" conversation [before the "two months later" gap]. Prior episodes are also referenced. This piece is standalone; if you like this, check out Transfigured Night, which is ongoing if less densely written.)

Either Giuseppe Tartini had six fingers or the devil had indeed written the Sonata in G Minor, as the fable said. When they'd gone to Dartmoor a while ago, he had urged John Watson to eliminate the impossible, but the sonata was as unsettling as the hound had been at Baskerville. With a piece like that, even Sherlock Holmes would have been hard-pressed to eliminate the impossible. There was trill after trill, and Tartini himself had discovered sum and difference tones. Surely that had to be more than merely –

Too sentimental, Sherlock decided. Trying to attribute the extreme difficulty of a violin piece to a supernatural origin was not only unnecessary, but wholly irrelevant and superstitious. He laid the A. E. Smith down gently, set the bow alongside, folded his hands under his chin, and sank into his armchair with a thump.

Even now, the devil was writing the music. Sherlock could still see the half-sipped cup of tea, the apple with crudely carved letters and a single bite, mere discordant notes meant to distract him from the tune as much as intrigue him by their composition... but how did the melody end? With a sudden descent, a fall, a decrescendo, since Moriarty had said as much. The more important question to answer was what the coda was, and thus how the piece as a whole would end, not just the melody he already knew.

Like Irene Adler had been many months ago, Moriarty was somewhat impenetrable – but whereas Adler had given him few clues of her own accord, Moriarty deliberately gave him too many and expected him to sort through the morass. That tapping. Was it a rhythm, a code? They'd started the impromptu meeting discussing Bach, but the finger-taps likely wouldn't indicate a series of notes. There would be a better system for that, something more complete.

It was something to do with those taps, though, and it was something Moriarty had said. "I did tell you," the madman had informed him, glee nearly dripping from his words and the toothy breadth of his smile, "but did you listen?"

He had indeed been listening. He would have noticed more, if there had been more to notice. He had the answer already within his grasp. He only had to figure out the riddle. He had lied to Moriarty, though. He enjoyed riddles. Along with composers like Tartini, unanswered questions made life worth living.

"I've been back for an hour, you know."

He glanced up, pulling his hands away from his chin. John was back, a reliably sturdy presence near the dining room table. Excellent. His flatmate had brought the documents to test a theory. Just the other day, a friend of Mr. Chatterjee's beneath them at Speedy's sandwich shop had come to him for help. Something mildly dull, of course, about a mysterious ringing in his ears, but it wasn't tinnitus; the audiologists had ruled out hearing loss, and it only happened when the man was at work in heavy industry.

But John had made him take the case as an apology of sorts, and only after Sherlock had suggested that the newfound client try hitting his head against a wall repeatedly to solve the problem. So the case was mostly John's responsibility anyway.

"Have you?" He didn't give John time to answer, springing to his feet. "You managed to get Mr. Russell's time sheet from Royal Park?"

John slung his rucksack on the table without preamble, rifling through it with a rustle of papers. The wait for the doctor to withdraw the requested document was maddeningly long, but Sherlock managed to avoid commenting - at least verbally.

"You don't need to glare at me like that for taking two whole seconds to pull out a piece of paper," John bristled at him, his words hard and snappish. "I'm the one doing all the legwork."

That comment was easily ignored. "And the record of when the plant machinery switches over?"

John didn't understand; Sherlock could hear it in the ensuing softening of the doctor's consonants as easily as he could tell from the words. "Yes, but he's not involved in the machinist operations. He's a ledger clerk for McVitie's." Despite this reluctance, John pulled out a spreadsheet, setting it on the table next to a series of chemical test tubes that sparked only occasionally.

It barely took a glance down at the spreadsheet to confirm his suspicions. "McVitie's may not be the culprit, but a plant there is." He tapped at the offending company. Tapping. What did it mean? His mind went back to the meeting with Moriarty. He had to analyze it further. There were answers within it, and if he could find them, then he could use them.

John laughed, more astounded than amused, wide-eyed. He was an appreciative audience. "All right, I know I've asked you this a thousand times before, but just how in the world did you – "

"This plant no doubt produces some sort of tone at a low amount of hertz. It's the only logical explanation, considering Mr. Russell apparently works in the offices that face one of the few plants that still works, and he hasn't any hearing loss. I looked on the McVitie's internal website to see how their headquarters was laid out, before you ask. They should choose a better password than 'Jaffa.' " He paused for breath. John was still staring, so he continued. "Tell Mr. Russell to get transferred to a department somewhere far away from any cooling towers or air compressors." The explanation was quick, rapid-fire, and unimportant. "After all, it's your case, John."

"Right, of course it's my case. Because you threatened Russell with bodily injury."

"I wouldn't have done it to him. Besides, it was only a suggestion."

John's jaw drew tight; his shoulders squared with anger. "A 'suggestion' that might have warranted an ASBO, if he hadn't come here looking for help." The shorter man pulled the spreadsheet back; Sherlock's hand flew off, relinquishing the paper. That seemed to surprise John, who let out a little laugh but took the paper again, far away from the chemicals, as if it – or Russell's case – mattered.

Only one thing mattered at present. Sherlock let himself fall into the couch. "Falling is just like flying," Moriarty had said, "except there's a more permanent destination." Where would they head? Would it be in the same direction?

John was still talking. It was distracting. "You know, sometimes, Sherlock, I don't even know why I bother to – "

"Thinking."

John was probably scowling at him; he was definitely muttering something unpleasant beneath his breath as he started for the stairs and marched stoutly up to his quarters, but Sherlock didn't need to know the specifics. It was as immaterial as which number planet Earth was from the sun, since apparently the sun was now at the center of the solar system. Or was it the universe?

But the center of Sherlock's universe right now was this puzzle that Jim Moriarty had left him, and he would solve it. Tartini had been bested by the devil, but Sherlock Holmes would not be so easily conquered. He was better than that. He was better than Moriarty. He would rise to the challenge, and he would win. A smile slid across his face, short and tight with tension, like a pizzicato pluck of a string.

He would play Jim Moriarty like a fiddle.