The sound of the rifle bolt was a period at the end of a sentence I had not yet finished reading. In the silence of our sitting-room, it possessed a clarity that was almost surgical. I felt the familiar, cold prickle of adrenaline—a ghost of the Afghan frontier—coursing through my veins. My hand moved instinctively toward the pocket where I kept my service revolver, but Holmes was faster.
With a movement so fluid it seemed a single blur of motion, he reached out and struck the lamp from the side-table. The glass shattered, and the room was plunged into a thick, coal-black darkness, save for the dying orange glow of the embers in the hearth.
"Get down, Watson!" he hissed.
I threw myself flat against the Persian rug, the smell of dust and old tobacco filling my nostrils. Beside me, I heard the heavy, labored breathing of Mycroft Holmes. Despite his corpulence and his usual sedentary habits, the elder Holmes had moved with surprising alacrity, shielding himself behind the sturdy mahogany of our sideboard.
"They will not fire yet," Mycroft's voice drifted through the gloom, surprisingly calm. "It was a warning, Sherlock. A punctuation mark. They want the parchment. They do not want the mess of a double homicide involving two members of the Diogenes Club and a decorated veteran of the Fifth Northumberland Fusiliers. The paperwork would be... tedious."
"Tedious?" Sherlock's voice came from the vicinity of the window-shutter. "You always were a master of understatement, Mycroft. To think that the British Government would employ such crude methods to recover a scrap of geometry."
"The Government knows nothing of this," Mycroft retorted. "This is the work of the 'Sub-Committee for Strategic Equilibrium.' A shadow within a shadow. Even I cannot always see the hands that pull their strings."
I felt a sudden, sharp draft of cold air as Holmes eased the shutter open by a fraction of an inch. I saw his silhouette—a lean, jagged outline against the grey smudge of the window. He was peering out into the yellow miasma of Baker Street, his magnifying glass in hand, as though he were examining a butterfly in a display case rather than a sniper on a roof.
"Seventeen," Holmes whispered.
"Seventeen what, Holmes?" I asked, my voice hushed.
"Seventeen steps, Watson. From the ledge of the opposite chimney stack to the gutter. I can see the indentation in the soot. Our friend is a man of precise habits. He has paced his ground. He is still there, crouched behind the parapet, waiting for a flicker of light."
"Sherlock, give me the paper," Mycroft commanded. "I can ensure its destruction. I can bury this matter so deep that even the history books will forget it."
"And what of the courier?" Holmes asked, his tone suddenly sharp. "A boy of twenty-five, Mycroft. Executed on a landing in Marylebone. Does he simply become a footnote in your 'Strategic Equilibrium'?"
"He was a variable that reached its limit," Mycroft said, and for the first time, I heard a note of genuine weariness in his voice. "He was warned in Vienna. He was warned in Paris. He chose to run. He chose to bring that infection here."
Holmes did not answer. Instead, I heard the scratch of a match. I gasped in terror, expecting a bullet to shatter the window-pane at any moment, but the flare of light was contained within the hollow of his palms. He was not lighting a pipe; he was holding the flame beneath the parchment he had taken from the dead man's hand.
"Holmes, no!" I cried.
"Peace, Watson," he murmured. "I am not burning it. I am observing it."
Under the heat of the match, the vellum began to undergo a subtle transformation. The non-Euclidean curves, previously a stark, coal-black, began to shimmer with a faint, iridescent violet hue. A second layer of notation appeared, interlaced with the first—a ghost-script of chemical formulae and Greek letters that seemed to dance in the flickering light.
"Synthetic ink," Holmes muttered, his eyes wide with a feverish intensity. "Iron-gall base, yes, but suspended in a solution of bismuth and cobalt. It is the variant used only by the Royal Mint for the printing of high-yield bullion certificates. But there is something else... a scent of almond. Cyanogen?"
"Do not inhale it, Sherlock!" Mycroft warned from the darkness.
Holmes blew out the match. The room returned to its stygian gloom, but the image of that glowing violet geometry seemed burned into my retinas.
"The ink is a message in itself, Mycroft," Holmes said. "It tells me that our assassin is not merely a soldier, but a technician. And this equation—$\lim_{x \to \infty} \left( \frac{\Phi(n)}{\Delta} \right) = \text{Regis}$—it is not a description of a physical structure. It is a calculation of a social one. $\Phi(n)$ represents the population density of a given urban center, and $\Delta$ is the rate of industrial output. The 'Regis' at the end... it is the Crown. The Monarchy."
"It is a predictive model," Mycroft sighed. "A way to calculate when a society becomes... unstable. When the weight of the many collapses the power of the few."
"And our courier?" I asked. "What was his role?"
"He was the bearer of the 'Friction Value,'" Holmes said, moving away from the window. "He carried the data that proved the equation was already reaching its tipping point. He was bringing the news that the collapse has already begun."
Suddenly, the silence of the street was broken by a sound that made my blood run cold. It was not a shot, but a rhythmic, metallic thumping—the sound of someone beating a heavy object against the cobblestones. It grew louder, more insistent, echoing through the fog like the heartbeat of a giant.
"The watchmen?" I suggested, though I knew the rhythm was wrong.
"No," Holmes said, his voice dropping to a low, dangerous register. "It is a distraction. A cadence meant to mask another sound. Watson, your revolver!"
I scrambled to my feet, my Jezail wound giving a dull, prophetic thud of pain as I rose. I drew my Webley and moved toward the door, but Holmes grabbed my arm.
"The floor, Watson! Look at the floor!"
In the dim light of the dying fire, I saw it. A thin, silver liquid was seeping through the gap beneath our sitting-room door. It moved with an unnatural speed, pooling on the carpet like mercury, yet it seemed to possess a purpose of its own. As it touched the wood of the floorboards, a faint, hissing sound arose, accompanied by a wispy, acrid smoke.
"Acid?" I whispered.
"Worse," Holmes said. "An atmospheric corrosive. It is eating the oxygen out of the room. Mycroft, we cannot stay here."
"The windows are watched," Mycroft said, his bulk shifting in the dark.
"Then we shall take the third option," Holmes declared. He turned to the fireplace and grabbed the heavy iron poker. With a strength born of desperation, he began to pry at the floorboards near the hearth. "The builders of 221B were fond of their shortcuts, Watson. There is a ventilation crawlspace that leads directly to the cellar of the empty house at number 223."
The silver liquid reached the edge of the rug, and the fabric began to dissolve into a black, bubbling sludge. The air in the room grew heavy and sweet, making my head spin. I felt a sudden, terrifying lethargy settling into my limbs.
"Quickly, Watson! Help Mycroft!"
I grabbed the elder Holmes by the arm, hauling him toward the gap Sherlock had opened in the floor. The massive man squeezed into the darkness with a grunt of exertion. I followed, my heart hammering against my ribs like a trapped bird.
As I lowered myself into the cramped, soot-stained crawlspace, I looked back one last time. The silver liquid had reached the dead courier's hand. As the corrosive touched the man's skin, his fingers—the ones that had held the 'Final Equation'—simply vanished, turning into a fine, grey ash.
"Holmes!" I gasped, reaching up for my friend.
Sherlock was the last to enter. He hung by his hands from the joists, his eyes fixed on the door. The handle was turning. Slowly, silently, the door to our sanctuary was being opened by an invisible hand.
"The equation remains, Watson," Holmes whispered as he dropped into the darkness beside me. "Even if the paper is gone, the logic is written in the air."
He pulled the floorboard back into place just as the door above us swung wide. There was a moment of absolute, terrifying silence, followed by a sound I shall never forget: the sound of a man—or something that breathed like a man—stepping into the room and walking with heavy, measured strides toward the very spot where we had been standing.
"The seventeen steps," I heard Holmes mutter in the pitch-blackness of the crawlspace. "He is counting them, Watson. He is counting them for us."