I Went Undercover as the Detective's Fiancée and Ended Up in His Bed

The telegram was, as always, brutally concise. Case. Come at once. SH. It was all the instruction I ever needed. I paid the cab driver and hurried up the familiar seventeen steps to the door of 221B Baker Street, the damp London air clinging to my coat. Mrs. Hudson let me in with a weary but fond smile, merely gesturing towards the stairs. The scent of acrid chemicals and strong tobacco was already seeping down.

I found him exactly as I expected: a whirlwind of manic energy at the center of a self-made storm. The sitting room was barely navigable. Newspaper clippings from The Times and The Morning Post were pinned to the mantelpiece with a jackknife, connected by a web of red string. Beakers bubbled over a Bunsen burner on the dining table, emitting a faint, foul-smelling vapor. He stood by the window, not looking out at the street, but staring at some invisible point in the middle distance, his tall, thin frame silhouetted against the gray light. He wore his dressing gown, and I knew from the dark smudges under his eyes that he hadn't slept in at least two days.

He didn't turn as I closed the door behind me. “The Mayfair Moth,” he said, his voice a low hum that vibrated with intellectual fervor. “That is what the gutter press would call our perpetrator, were they to catch wind of it. A fitting, if overly romanticized, moniker.”

He finally turned, and his eyes, the color of storm clouds, pinned me in place. The intensity of his focus was a physical thing, a force that always made the air in the room feel thin. It was a look I knew well, the look that preceded the unraveling of some impossible knot, and it never failed to send a disloyal flutter through my stomach. I suppressed it ruthlessly. Professionalism was the armor I wore in this room.

“There is a predator moving through the drawing rooms and private clubs of London’s elite,” he continued, gesturing to the chaotic display. “The method is consistent. Seduction, followed by a period of idyllic intimacy, and then the demand. Blackmail. The victims are powerful, wealthy, and, most importantly, married. Sir Reginald Crowley of the Foreign Office, Lady Beatrice Althorpe… the list is growing. They pay, of course. The scandal of exposure would be far more ruinous than the loss of a few thousand pounds.”

He picked up a photograph from the clutter, a candid shot of a smiling, handsome man. “The common denominator is not the victims, but the nature of the secrets they keep. The Moth does not merely threaten to expose infidelity. It uncovers far deeper, more dangerous truths that the affairs were meant to conceal.” His gaze was sharp, analytical, yet something in their depths held a fire that was purely him. It was the part of him I admired most, and the part I had to work hardest to ignore.

Our first lead was a name whispered by one of Sir Reginald’s terrified associates: Julian Croft, an art dealer whose gallery was hosting a private viewing that very evening. It was the perfect hunting ground.

Two hours later, I stood beside Sherlock in a room dripping with wealth and feigned sophistication. The air was thick with expensive perfume and the low murmur of gossip. He had changed into a perfectly tailored dark suit that made him look severe and out of place, a predator who had wandered into a flock of peacocks. I wore a simple but elegant dark blue dress, the kind that allowed one to blend in. We moved through the crowd, a glass of untouched champagne in my hand, my senses on high alert.

“Notice the patterns,” Sherlock murmured, his voice low enough for only me to hear. “Lady Althorpe is avoiding Lord Harrington, whose wife is speaking with the very man Sir Reginald was rumored to be in business with before his… misfortune.” His eyes scanned the room, cataloging every glance, every strained smile.

Then we saw him. Julian Croft. He was standing near a large, abstract canvas of violent reds and blacks, holding court. He wasn't classically handsome, but he possessed a magnetism that was almost a physical force. He had dark, intelligent eyes, a smile that seemed both genuine and predatory, and a way of leaning in when he spoke to someone that created an instant, conspiratorial intimacy. He moved with a languid grace, his hands expressive as he discussed the art, and I watched as a woman—the wife of a prominent barrister—blushed under his focused attention. He was the moth, and they were all drawn to his flame.

I watched him work, a cold knot of understanding forming in my stomach. This wasn't just about blackmail. It was about power. The power to see a person’s deepest insecurities and desires, and to use that knowledge to own them completely. It was a cruel, intimate violation.

As if he’d plucked the thought directly from my mind, Sherlock spoke, his gaze still fixed on Croft. “You find his method repellent, yet you cannot deny its effectiveness. The idea of being so thoroughly understood, of having your most private vulnerabilities laid bare and then accepted by another… you see the appeal, even in its perversion.”

His words struck me with the force of a physical blow. My breath caught in my throat. It was precisely what I had been thinking—the horrifying, secret allure of being completely and utterly seen. A hot flush crept up my neck, a mixture of indignation and something else, something shamefully thrilling. He hadn’t even looked at me. He had simply reached into my mind and pulled out a thought I barely knew I was having. I felt exposed, stripped bare right there in the middle of the crowded gallery. It was infuriating, this casual violation of my own mind. And yet, the accuracy of it, the sheer, brilliant perception, sent a deep, dangerous tremor straight through me.

I swallowed hard, forcing myself to hold his gaze, refusing to give him the satisfaction of seeing me look away. "Your observations are noted," I said, my voice tight and clipped. He gave a faint, almost imperceptible smile, a flicker of acknowledgment that he had hit his mark, before his attention returned to Croft. The rest of the evening passed in a blur of strained pleasantries and covert observation. The air between us was thick with what he had said, with what he had seen inside me. Every time he moved closer to murmur an observation, I felt the heat of his body as a brand.

The ride back to Baker Street was silent. I stared out the window of the hansom cab, watching the gaslights smear across the wet glass, acutely aware of Sherlock's leg resting a mere inch from my own. The space felt impossibly small, charged with a tension that had nothing to do with the case and everything to do with the man beside me.

“It will not be enough,” he said suddenly, his voice cutting through the rhythmic clatter of the horse’s hooves on the cobblestones.

I turned to look at him. His profile was sharp in the flickering light, his expression grimly focused. “Observing him from the periphery is useless. Croft doesn’t choose his victims at random. He cultivates them. He builds a world of perceived intimacy, of shared secrets. To catch him, we cannot be outsiders looking in. We must become the next potential masterpiece in his collection.”

A knot of apprehension tightened in my stomach. “What are you proposing?”

He turned his head, his gray eyes locking onto mine in the gloom of the cab. The intensity there was absolute, leaving no room for argument. “I have already sent a wire to Mycroft. He is arranging a suitable flat in Mayfair and a backstory. Lord and Lady Ashworth. Old money, but new to London society. And, most importantly, newly engaged.”

The words hung in the air between us. Engaged. The single word seemed to suck all the oxygen from the small carriage. My mind reeled, trying to process the implications. It wasn't just sharing a workspace or a cab. It was a flat. A life. A performance of intimacy so complete it would have to fool a master of emotional manipulation.

“You will be my fiancée, Keri,” he stated, not as a question, but as a fact. “It is the only way. He preys on the vulnerabilities of a new, passionate union. The public displays of affection, the private language of lovers… we must be flawless.”

A jolt, sharp and electric, shot through my entire body. It was a surge of pure, unadulterated panic mixed with a terrifying, subterranean thrill. My heart began to hammer against my ribs, a frantic, trapped thing. All the carefully constructed walls, all the professional lines I had drawn and patrolled for years, were about to be systematically dismantled. The thought of it—of his hands on me, of looking into his eyes and pretending it was love, of willingly blurring that boundary until neither of us could be sure where the performance ended and the truth began—was the most dangerous idea I had ever heard. And I knew, with a dreadful certainty, that I was going to say yes.