Author's Note: Hello! This is a crossover of Sherlock Holmes and Harry Potter. While I do love to read the Holmes stories, this version is actually inspired by the brilliant BBC series, "Sherlock." This story takes place a year before Harry Potter comes to Hogwarts, so expect to see the same professors and a few familiar, if younger, faces. Cheers!

Hogwarts: A Mystery

Chapter One: First Impressions

"Wait up!" I gripped my broom tightly as I ran up the lawn toward the castle. A throng of gold and burgundy surged ahead of me, holding aloft the man of the hour – Arthur Digby.

"DIGBY! "DIGBY!" The students chanted enthusiastically as they passed through the great wooden doors into the hall. The blonde seventh-year waved his hands in triumph.

As I passed into the entryway, a low voice behind me stopped me in my tracks. "What's all this, then?" I turned to see a lanky, dark-haired boy sitting on the floor, leaning against the wall. He had a large old book in his lap and was surrounded by vials of various liquids. Odd place to be doing potions homework, I thought. The boy, who appeared to be a sixth or seventh year student, wore no cloak or sweater – just his white uniform shirt, sleeves rolled to his elbows, and his blue and bronze tie hung loosely around his neck. He raised an eyebrow and looked expectantly at me.

"How could you not know?" I was dumbfounded. "Arthur Digby's just won Gryffindor the first quidditch match of the year!" I was giddy with excitement – the first game of the year always gave me a rush.

The boy looked past me to the crowd celebrating in the entryway. He studied Digby a moment and turned to me. "Too bad he cheated."

My mouth hung wide in shock and the crowd of revelers grew silent behind me as the boy resumed reading the giant tome perched on his knees. Students started whispering as Arthur Digby pushed his way toward the boy, who had once again turned his attention to his book.

"What did you say?" Digby was slightly shorter than average height, though of muscular build. He was known throughout the school for being incredibly sure of himself, which I suppose is a nice way of saying he was a bit of an ass. But three straight years of winning Gryffindor the Hogwarts Quidditch Cup gained him certain allowances.

"I said you cheated, Digby." The Ravenclaw looked up, stone faced, at Digby, whose ears were quickly becoming a bright shade of pink.

I decided to interject, on behalf of Digby and the rest of the team. "But if you weren't there, how could you possibly know something like that?"

The dark-haired lad smirked at me, pointing to Digby's arm. "A twitch. Right there, do you see it?" I looked at Digby and, sure enough, noticed a spasm, slight and intermittent, in the seeker's left hand.

"Surprised you could catch the snitch with that arm, Digby."

Digby crossed his arms to hide his malady. "A nervous habit. I'm right-handed anyway," he quickly added. Arthur's face was growing red and the twitch seemed to be getting worse.

"You've thrown enough balls of parchment at me in Charms for me to know that's a lie. Besides, judging by the rate of acceleration, I'd say the twitch is a recent development. Say, the last five minutes or so." The boy closed his book. "It's getting worse, Digby. I'd get that looked at if I were you."

A pair of red-headed second-years had been standing nearby. "That's all well and good," said one. "But what's a twitch got to do with him cheating?" finished the other.

The Ravenclaw finally stood, towering over Digby, looking down his long nose at the boy. "Oxygenum Liquium. More commonly known in the quidditch world as 'Liquid Agility.'" He paused for a moment, looking expectantly at the group of students. Finally he gave an exasperated sigh. "It oxygenates the blood, increasing the rate of aerobic respiration in the muscle tissue."

The crowd stared at him blankly. "It means you don't get tired! Really, don't any of you read books?"

Digby stepped toward the boy, jamming his finger in his chest. "That's a damn lie. You don't have any proof."

The tall Ravenclaw smirked again. "On the contrary, we have a physical symptom of a potion poorly made." He gestured to Digby's arm. "I'm guessing you ran out of powdered snakeskin, a common problem." The boy began to pace, rubbing his hands together as he did so. "And common problems, as we all know, call for common solutions. In this case, wolfsbane. Everyone knows that these two ingredients have the same basic properties. Except they don't. Unlike powdered snakeskin, wolfsbane must be diluted with extract of monksroot when combined with gilly water – the main ingredient in any good agility potion. Because, as every first-year potions student knows, wolfsbane and gilly water can have potentially lethal effects when ingested simultaneously."

The boy stopped in front of Digby and crossed his arms. "I'm going to guess you have about thirty-five minutes until you're seizing on the floor, after which I'm fairly certain your heart will explode in your chest." The color instantly drained from the young Gryffindor's face after hearing the boy's matter-of-fact diagnosis. "I'd get yourself to Madam Pomfrey at once. If this is how you make potions, I'd hate to see you perform a cardionixus charm on yourself – your brain might shut down."

At that moment, Professor McGonagall emerged from the crowd, pushing a few horrified first-years aside to get to Digby, who, by this time, was as white as a sheet. "Mr. Digby," she said when she finally made her way to the boy, "I am appalled at your actions. Putting your own life at risk for the sake of a quidditch match? Never have I seen any of my students behave so foolishly." She turned. "Mister Weasley, kindly escort mister Digby to Madam Pomfrey. After which, mister Digby, you will report to Mr. Filch." Arthur flinched, either at Filch's name or due to the increasing severity of his potion's side-effects, I couldn't tell.

McGonagall addressed the crowd. "In light of this news Gryffindor is hereby disqualified from the game. Tournament points will be awarded to Hufflepuff. Needless to say, fifteen points will be deducted from Gryffindor for poor sportsmanship." She then turned to the young Ravenclaw who, during all the excitement, had sat back down and continued reading his book as if nothing had happened. "As for you, mister Holmes, for exhibiting all of the qualities befitting a Ravenclaw I am awarding ten points to your house."

With that, Professor McGonagall turned to follow Digby, who was now being escorted up the grand staircase by the two young red-headed twins. As the crowd dispersed, angry glares were cast at the young Ravenclaw, who in his studying took no notice.

I stood there a moment, feeling a mixture of crushing defeat and… and odd sense of wonderment. I turned to the boy. "That was brilliant. All that from a twitching hand?"

He spoke, not looking up from his book. "It was basic potions. Arthur Digby is a pompous imbecile who wouldn't know a goblin from a grindylow." He snapped the book closed and began packing his potions into his bag.

"Holmes, is it?" I stuck out my hand. "Nice to meet you, I'm-"

"John Watson." He stood, looking at my hand, but not taking it. "Fifth-year Griffyndor student, beater for the quidditch team, rubbish at potions, but exceedingly skilled in charms."

I stared at the boy. Surely we hadn't met before. And, if we had, I felt terrible that his name had slipped my memory.

Holmes slung his bag over his shoulder. "If you'll excuse me, Watson, I have some work to do in the library. Perhaps we'll see each other again sometime."

With that, he started off toward the staircase, leaving me to stand dumbfounded in the entrance.

"O-Levels are coming up this year, John. All I'm saying is that it might be beneficial to ask for a little extra help."

It was two o'clock and Sarah Sawyer, Oliver Wood, and I sat at one of the tables in the Great Hall. Oliver, while pretending to study for our upcoming arithmancy exam, was too busy drawing up quidditch plays to pay attention to our conversation.

"I'm fine on my own." I absently turned the pages in my book as Sarah glared at me from across the table. "Besides, what would I do with potions, anyway? I'm a lost cause, Sarah."

Refusing to leave it at that, Sarah shoved a piece of parchment at me across the table. "Just look at it, John."

Ravenclaw Mentorship Program Application

I sighed. "I don't need a mentor, Sarah."

Oliver looked up from his doodles. "I'd listen to her, John. You'll never hear the end of it."

I reluctantly picked up the paper. Attached was a list of the available mentors – all Ravenclaws, obviously. As I scanned the list, one name stood out.

Sherlock Holmes.