Pale blue steam drifted through the Potions classroom at Hogwarts.
Everyone was giving it their all for the vial of Felix Felicis that Professor Slughorn had produced.
As they worked from Advanced Potion-Making to brew the Draught of Living Death, they kept glancing over to see what everyone else was doing.
That was both a blessing and a curse of Potions class.
Because as long as you bothered to look, you could see exactly what anyone else was up to.
But seeing was one thing. Copying it was another matter entirely.
Hermione, for instance, was clearly in the lead.
Her potion was already close to the "smooth, treacle-like liquid" the book described as the ideal state at the halfway point of brewing.
Ron had copied her movements step for step, almost the whole way through, and yet his potion had turned a bizarre shade of purple, and the smell didn't seem quite right either.
Just then, a rather discordant voice rang out: "Sir, I daresay you must have known my grandfather, Abraxas Malfoy?"
No one else paid it any mind—only Sherlock and Harry glanced up.
Slughorn was passing the Slytherin table, and Malfoy had seized his chance to say that.
Unfortunately for him, though Slughorn did answer, he didn't so much as slow his pace.
"I heard he'd died. Very sorry to hear it, though it wasn't unexpected, at his age, with dragon pox on top of it…"
Harry couldn't help but suppress a smile.
Clearly, Malfoy was after some special treatment of his own.
If not the kind he and Sherlock got, then at least something like what Zabini received.
Unfortunately, Slughorn was far too pragmatic for that.
If Malfoy's grandfather had still been alive, perhaps he'd have gotten a second look.
But now—if he wanted to win that vial of Felix Felicis, he'd have to rely on his own wits.
Which, of course, was never going to happen.
Not with Sherlock and Hermione on their side of the room.
Thinking that over, Harry found he wasn't in much of a hurry after all.
He bent his head back down to his textbook.
One look, though, only left him more irritated.
His eyesight for close work wasn't good, and he had to strain to make out the instructions buried under the scrawled handwriting of the book's previous owner.
What frustrated him more was that, for some reason, that fellow disagreed with the book's instruction to slice the Sopophorous bean, and had scribbled a different note instead.
[Crush with the flat of a silver dagger, releases more juice than slicing]
Harry blinked, and without thinking, turned to ask Sherlock about it.
A knife was already in front of his face.
Harry stared at Sherlock in surprise—he hadn't even said anything yet!
Sherlock pointed at the line of handwriting, then spoke.
"Do as the book says."
"Sherlock?"
Harry was even more surprised to hear that.
"Trust me on this."
Bewildered, Harry took the silver dagger and glanced around.
Hermione, meanwhile, looked rather troubled, her eyes never leaving her own potion for a moment.
After reaching the "smooth, treacle-like liquid" stage, once the bean juice was added to the cauldron, it was supposed to turn a pale lilac.
But Hermione's cauldron was still deep purple.
As for Sherlock, his progress seemed even slower than hers.
Harry had no idea what Sherlock was playing at. Taking the dagger, he first sliced at the bean out of habit.
He found the Sopophorous bean was maddeningly hard to cut, and even once opened, it barely released any juice.
Catching himself, he hurried to press the flat of the blade against the bean instead, following the note's instructions.
To his astonishment, the bean immediately oozed out a great deal of juice.
Harry could hardly believe it—that shrivelled, dried-up-looking little bean held that much liquid inside it.
"Sherlock, this—"
Sherlock had been watching Harry's every move, and what he saw only confirmed what he'd already worked out.
"Keep going," he said, with a faint smile at Harry. "Just follow what the book says."
With Sherlock backing him up, Harry naturally felt more confident.
Once he added the bean juice to the cauldron, it turned the pale lilac the book described, exactly as promised.
At that moment, Harry's admiration for both Sherlock and the book's mysterious owner grew even further.
For the rest of the process, he followed the instructions to the letter.
Before long, his cauldron held something unlike anything else in the room clear as water.
It put Harry in tremendous spirits.
This dungeon classroom had never seen anything like it before.
Even Hermione's potion remained stubbornly purple, only a shade or two lighter than before.
"Sherlock, you—"
Noticing Sherlock seemed to have simply been watching him work the whole time, Harry felt his face go a little red.
The book's former owner had given brilliant instructions—that much was certain.
But if Sherlock had spotted as much while Harry himself still had doubts, he'd clearly seen something too.
And with Harry's own work as an example to follow, Sherlock, with his skill, would surely have made something even more impressive if he'd simply copied it.
And yet Sherlock hadn't done that.
"One of you is enough," Sherlock said with a slight smile, pointing at himself. "I'm just here for the fun of it."
Harry understood at once.
Sherlock had said something like that before—that he had no need for the credit of solving a puzzle. What he enjoyed was the process itself.
This was no different.
During all this, Hermione had noticed Harry's performance was unlike anything she'd seen from him before.
But when Harry mentioned that his method differed from the textbook, she shook her head stubbornly, calling it cheating, and insisting he ought to follow the book's instructions instead.
Well, if that was how she wanted it, there was nothing more Harry could do.
He'd given her the chance and she hadn't taken it!
An hour sounds like a long time, but when you're truly absorbed in something, it goes by in a flash.
Soon, Professor Slughorn's voice rang out.
"Time's up—stop stirring, please!"
Slughorn then began making his way slowly between the tables, checking each cauldron in turn.
He offered no comments, only stirring occasionally, or leaning in for a sniff.
Whether by design or not, he came last to the table where Sherlock, Harry, and the other two sat.
He gave a wry little smile at the tar-like mess in Ron's cauldron.
Then, glancing at Hermione's potion, he nodded approvingly.
Noticing Sherlock's half-finished brew, he shook his head with a touch of regret.
But when his eyes fell on what was in Harry's cauldron, his face lit up with unmistakable delight.
"An undisputed winner!"
Slughorn announced loudly to the entire dungeon.
"Excellent, simply excellent, Harry!
"Goodness, you've clearly inherited your mother's gift—Lily was every bit this deft in Potions back in her day!
"Here, take it—a promise is a promise, one bottle of Felix Felicis, and use it well!"
Harry tucked the small vial of golden liquid into his robes without quite thinking, his feelings a tangled mess.
Malfoy and several other Slytherins wore looks of pure irritation.
The Ravenclaws, for their part, looked simply astonished.
"How did you do that?"
Ron couldn't help asking Harry as they left the dungeon.
If it had been Sherlock or Hermione who'd pulled this off, he wouldn't have batted an eye.
But he knew exactly what level Harry's Potions skills were at.
Harry looked over at Sherlock.
Sherlock gave a faint smile. "Simple. Harry had it in him all along."
The young witches and wizards nearby, who'd all been listening with their ears pricked up, looked utterly stunned.
Hermione and Ron were equally floored.
Hadn't they agreed not to lie?
Wait—hadn't the actual agreement been that lying was permissible when necessary?
So this counted as necessary, then?
It wasn't until lunch that Harry finally told the other three the whole story.
That, too, had been Sherlock's request.
Like with a case, every time Harry retold it, he noticed some new detail.
"Hermione, I suppose you think I cheated?"
Once Harry finished, noticing Hermione's darkening expression, he asked her directly.
Hermione took a deep breath. "That's right—you didn't do it entirely on your own, did you?"
"Hermione, that's not fair."
Ron jumped in to smooth things over. "Harry just used a different method than the rest of us. Might get good results, might blow up in his face—right?
"Harry took the risk, so he got the reward.
"Come to think of it, Slughorn might well have handed that book to me instead.
Except, well—nobody ever wrote in my copy. Judging by page fifty-two, it looks like somebody was sick on it—"
Hermione was still frowning when, a moment later, she suddenly seemed to remember something and cried out.
"Wait—you all remember the diary, don't you?"
"Diary? What diary?"
Ron was still catching up, but Sherlock and Harry had already understood.
Harry lowered his voice. "Hermione, you know it's not like Riddle's diary—it's just an old textbook someone scribbled in."
"But you still did what it told you to?"
"I just tried a couple of the little tricks written in the margins.
"Honestly, Hermione, there's nothing suspicious about it, and besides, even Sherlock—"
Before he could finish, Hermione had already yanked the copy of Advanced Potion-Making out of Harry's bag and raised her wand.
"Specialis Revelio!" she rapped smartly on the cover, incanting.
Nothing happened.
The book remained a book worn, grubby, its corners curling.
"Satisfied?"
Harry was starting to lose his patience. "Were you expecting it to do a somersault or something?"
"Looks fine, I suppose."
Even so, Hermione still eyed the book suspiciously. "I mean, it really does look like just a textbook—anyway, what did you just say?"
"What he meant to say is that it was my idea he do it that way."
Sherlock, who'd been watching Hermione's little performance the whole time, spoke up.
"What?"
"What?!"
Hermione and Ron both stared at Sherlock in shock.
"I was just about to say so," Harry said, a little helplessly, "but you'd already snatched the book away."
"Well, if even Sherlock says it's fine, then it really must be fine!"
Ron shrugged at that. "Guess it wasn't really a risk after all, then."
Hermione, though, looked at Sherlock in confusion. "Sherlock, do you actually know what's going on here?"
Sherlock smiled faintly, took the book back from Harry, and flipped it open to the inside back cover, pointing.
With Sherlock indicating it, Harry, Hermione, and Ron all saw a line of writing.
Just like all the other notes crowding the book's margins.
[This book is the property of the Half-Blood Prince]
For the next several Potions lessons, whenever the Half-Blood Prince disagreed with Libatius Borage's textbook, Harry followed the Prince's suggestions instead.
By the end of the week, Professor Slughorn couldn't stop praising Harry's ability, saying he'd rarely taught a student with such talent.
The last student that talented, he added, had been Harry's own mother, Lily Evans.
Neither Ron nor Hermione was particularly pleased about any of this.
Harry, for his part, brought the book out to share with the two of them.
But Ron couldn't decipher the handwriting nearly as fluently as Harry could, and he couldn't very well keep asking Harry to read it aloud for him not without drawing attention.
As for Hermione, she stubbornly stuck to the textbook's official instructions.
The potions she brewed that way turned out passable enough, but nowhere near as good as the ones made following the Prince's notes.
Sherlock, naturally, was happy to try the Prince's suggestions himself.
But as before, he preferred to let Harry take the credit.
And so, Hermione and Ron's tempers only grew worse by the day.
Until one Saturday evening.
With so much homework piling up, Harry still hadn't found time to give the battered old Advanced Potion-Making a proper read from cover to cover.
But he had flipped through it roughly, start to finish, and found that this Half-Blood Prince had scrawled notes on nearly every single page.
And those notes weren't only about potion-making, either.
Some of them looked like spells the Prince had invented himself.
"Might've been a woman."
Hermione said it impatiently, in the common room, as Harry read some of the spells out loud to Ron.
"Could've been a girl, even because I don't think that handwriting looks like a boy's. More like a girl's, if you ask me."
"He calls himself the 'Half-Blood Prince,'" Harry said. "How many girls go around calling themselves princes?"
Hermione seemed to have no answer for that, and turned instead to the person beside her. "Sherlock, what do you think?"
Sherlock, who had long since finished all his homework, was at that moment holding the Prince's book, studying the spells written inside.
"I think Harry's right."
Sherlock said it without even looking up.
"And honestly, whoever this Prince is, he's a good deal better at teaching than our actual professor is at lecturing.
"If Snape taught the way this book does, I daresay a great many people's O.W.L. results in Potions would improve considerably."
"See, even Sherlock thinks the Prince—Harry, Hermione, what's wrong with you two?"
Ron, mid-sentence, suddenly noticed that both Harry and Hermione's expressions had changed.
"Sherlock, you—you mean to say—" Harry stammered.
Hermione stared at Sherlock with an expression of pure disbelief. "This Half-Blood Prince is—"
"Obviously."
Sherlock set the book down and gave the three of them a faint smile.
"Severus Snape."
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