The castle at night always had its own rhythm—whispers through stone, portraits snoring in gilded frames, staircases shifting with sighs like restless giants. Harry moved easily under the Invisibility Cloak, footsteps silent on the flagstones. Beside him, Hermione clutched the Marauder's Map, her eyes darting from the parchment to the shadowy corridors.
Hermione whispered, "I still don't understand how you do this every night without getting caught. My heart's racing."
Harry smirked faintly. "You get used to it. Hogwarts looks different at night… less like a school, more like a puzzle waiting to be solved."
Every time footsteps echoed ahead, they melted into alcoves, holding their breath until Filch, or Peeves, or a patrolling prefect passed. Hermione admitted quietly, "It's beautiful, though. Like the castle is alive."
Harry only nodded.
As they neared the dungeons, the sound of a sharp, familiar voice froze them in place.
"Caught you red-handed, Weasleys."
Harry and Hermione exchanged a glance. Slowly, they crept closer, cloaked and silent, until they saw the scene unfold: Professor Snape had cornered Fred and George. His black eyes glittered like oil lamps, and his voice was venomous.
Fred, ever the actor, raised his hands. "Professor Snape, you wound us! Do we look like thieves?"
George added innocently, "We were just… wandering."
Snape sneered. "My stores have been raided. Lacewing flies, boomslang skin, fluxweed… all missing. Ingredients for only one potion I can think of, Polyjuice."
Hermione whispered almost same time, "Polyjuice."
Snape stepped closer to the twins, his robes billowing. "You think I don't know what game you're playing? I ought to drag you both to the Headmaster this instant."
Fred grinned, though sweat trickled down his temple. "Poly-what now? Sounds like a fine drink. Does it come in cherry?"
George coughed into his sleeve, "Or possibly lime?"
But Snape wasn't laughing. His wand twitched in his hand as though he were seconds away from hexing them.
Under the cloak, Harry's thoughts raced. Polyjuice Potion. It explained too much. The mysterious attack in the corridor, the shadows that didn't quite fit… If someone had access to Polyjuice, they could be anyone in Hogwarts.
Hermione gripped his arm hard, her voice a tremor. "If someone's brewing Polyjuice, Harry, it means they're hiding in plain sight. Anyone could be the attacker."
Harry whispered back, "Or they already are."
Snape finally lowered his wand but not his suspicion. "Go. Before I change my mind. But mark my words, Weasleys, if one more thing goes missing, I will see you expelled and hundred point from Griffindor."
The twins scuttled away, muttering about injustice and greasy-haired tyrants.
When the corridor was empty again, Harry pulled Hermione further into the shadows.
"This isn't just about Fred and George making mischief," Harry murmured. "Someone is brewing Polyjuice inside this castle. And if Snape's right, they already have everything they need."
Hermione looked up at him, her eyes wide in the torchlight. "Then we're not dealing with a student, Harry. We're dealing with a spy."
The fire in the hearth crackled softly, throwing long shadows across the empty Gryffindor common room. The hour was late, and everyone else had already gone to bed. Harry and Hermione sat together on one of the old red sofas, their cloaks still damp from sneaking through the drafty corridors. The Marauder's Map lay open across Harry's knees, the ink glowing faintly in the firelight.
"Harry, you don't understand—Polyjuice doesn't last forever. An hour at most. Whoever is brewing it needs to drink it regularly. That means we can track them, if we look carefully enough."
Harry nodded, his mind racing. "This isn't some student sneaking into the girls' dormitory or wanting to copy homework. This is someone hiding in plain sight."
Hermione clutched his arm tighter, her eyes wide. "Exactly. A Polyjuice disguise needs hair or something from the person they're imitating. If the real student was dead, it wouldn't work. That means—"
Harry finished grimly, "They've got someone locked up somewhere in this castle. Someone alive."
The two of them exchanged a silent, horrified look. The corridors seemed to breathe heavier around them, as though Hogwarts itself knew of the conspiracy hiding in its walls.
Hermione straightened, her voice fierce. "It has to be an adult, Harry. A student doesn't need Polyjuice—why hide themselves here, when they're already allowed to attend classes? No, this is someone older, an intruder or enemy masquerading as a student. That would explain the attack on you."
Harry's eyes narrowed, his voice low. "So someone… someone older, pretending to be a kid again. Waiting, watching, until they can strike."
Hermione nodded rapidly. "Yes. And if we're clever, we might catch them. Look for patterns. A student who excuses themselves at exact intervals, who leaves class for a few minutes every hour. That's when they drink the potion."
Harry looked down at the Marauder's Map in her hands, the ink glowing faintly. "Then we watch. The map can't be fooled—it'll show us the real name of the person walking around. Even if they look like someone else."
Hermione's eyes widened, realizing the full weight of the weapon in their possession. "Yes, Harry. If someone's hiding as a student… the map will betray them. We just have to be patient."
Harry's jaw clenched. He could almost feel the Force humming through his veins, black lightning itching at his fingertips. "If this intruder is the one who cursed me… I'll find them. And when I do, they won't live to see the next sunrise."
Hermione gave him a sharp look but didn't scold him. She knew better by now than to argue with the steel edge in his voice. Instead, she placed a hand gently on his shoulder. "Then let's find proof first. Whoever they are, we'll expose them."
Harry gave a short, curt nod. "Fine. But once I know their face, Hermione, they're finished."
The Great Hall was buzzing that morning, spoons clattering against bowls and owls swooping down in lazy arcs. Harry sat across from Hermione and Neville, the Marauder's Map discreetly spread between the juice jug and toast rack. Hermione's quill was tapping lightly against her plate as she scanned the parchment, whispering names under her breath. Neville leaned closer, his brow furrowed, trying to keep up.
Fred and George dropped down opposite them with identical grins, their plates piled high with bacon and kippers.
"Now, now," Fred said, stabbing a sausage with his fork. "What do we have here?"
"Looks suspiciously like our property," George added, peering over. "And here we thought you were only borrowing it, Harry."
Harry didn't look up immediately; his finger traced the tiny ink footprints of Gryffindors moving about the tower. Then he raised his eyes, calm and measured.
"Relax. But I've found a use for it that matters more than sneaking into kitchens."
That caught their interest. Both twins leaned in, lowering their voices.
"A use?" George asked.
"Do tell, oh Chosen One," Fred added.
Harry glanced at Hermione, then back at the twins. His tone dropped into a whisper.
"There's an intruder in Hogwarts. Someone's here who shouldn't be. They're using Polyjuice Potion to stay hidden."
The twins straightened, their joking manner evaporating. George's fork clattered back onto his plate.
"Polyjuice?"
Fred's grin twisted into something darker. "That would explain why Snape had a go at us yesterday. Said we'd nicked from his stores."
Hermione nodded gravely. "Snape wasn't wrong about the missing ingredients—but whoever it was wanted the core components. That means they've been brewing for weeks."
The twins exchanged a look, then Fred muttered, "And here we are, nearly thrown into detention for something we didn't even do."
Harry slid the map a little closer. "That's why we're checking. If someone's disguising themselves as a student, the map will still show their real name. The problem is—" he tapped at the parchment, "—we don't know every student in Hogwarts. I can spot all the Gryffindors, but once we move into Ravenclaw, Hufflepuff, and especially Slytherin territory… too many unfamiliar names."
George squinted at the parchment. "Well, that's awkward. The lot of them blur together, don't they?"
Neville mumbled, "I barely remember half the Slytherins in our year."
Fred groaned, dragging a hand through his hair. "And you expect us to sit here cross-referencing names? That'll take weeks."
Harry's eyes hardened. "Better weeks than ending up hexed in the back. Whoever tried to curse me last week nearly got through my armor. If they can do that to me, they can do it to anyone."
The words sank in, and for once, the twins looked serious.
Hermione pulled the map closer again. "Then we'll start with Gryffindor. One by one. We know everyone here—faces, names, even habits. If there's a mismatch, we'll spot it. After that, we move outward."
George gave a long sigh but nodded. "Fine. But if we're doing this, we're doing it properly."
Fred smirked again, though it lacked some of its usual cheer. "Imagine the headlines: Weasley Twins Save Hogwarts. Has a nice ring, doesn't it?"
Harry didn't smile. His fingers pressed into the table edge as his eyes swept the map. He was already thinking of darker possibilities—the Polyjuice-user wasn't just hiding. They were plotting.
The investigation with the Marauder's Map turned out to be far slower than Harry expected. Meals at Hogwarts weren't regimented: some came early to breakfast, others staggered in late, and foreign students from Beauxbatons and Durmstrang kept to their own rhythms. That meant the map never showed everyone at once, making the process of elimination painstaking.
Harry, Hermione, Neville, and the twins sat huddled over the parchment whenever they could, whispering names and checking clusters. The map's delicate ink shuffled constantly, footprints drifting in and out of rooms across the castle.
"Merlin's beard, at this rate, we'll be old men before we've checked them all," Fred muttered.
"Speak for yourself. I plan to be young and charming forever," George quipped, though his tone was weary.
Hermione frowned, tapping her quill. "The problem isn't just numbers. It's that we don't know every face. Especially the visitors."
That truth gnawed at Harry more than he let on. Beauxbatons girls were nearly indistinguishable in their powder-blue uniforms, and Durmstrang students rarely mingled beyond their own group. Any one of them could be a disguised enemy.
Harry's suspicion shifted. Percy was no longer at the center of it. He thought carefully about the man's pompousness, his Ministry robes, his authority granted through Barty Crouch's illness. Percy didn't need Polyjuice. With Ministry backing, he could walk anywhere without question.
But Igor Karkaroff—he was another matter. Sirius had warned Harry about him in one of their late-night mirror conversations: Karkaroff sold out his fellow Death Eaters to escape Azkaban. Never trust a man who betrays to save his own skin.
That memory festered now.
What if the Durmstrang headmaster wasn't just overseeing his students? What if he was plotting?
Harry watched the Durmstrang ship anchored in the Black Lake, its tall masts jutting into the grey sky. He imagined a Hogwarts student locked somewhere in its bowels, their hair clipped every few days to fuel Polyjuice brews. His stomach churned at the thought.
He muttered darkly to Hermione one evening by the fire, "They could have kidnapped someone. Kept them alive, hidden on that ship. That's all they'd need—a constant supply of hair."
Hermione paled, but her rational side still pressed back. "That's… possible, but dangerous. Surely someone would notice if a student went missing for that long?"
Harry's eyes narrowed. "Not if they picked carefully. Not if they knew exactly who wouldn't be missed."
The paranoia only grew. Harry didn't trust professors either—not even Dumbledore. He thought of Snape often, stalking the dungeons like a bat. Snape had the knowledge, the subtlety, and the motive to brew Polyjuice. He had already accused Fred and George of stealing from his stores, but what if that was misdirection?
Every time Harry studied the map, his mind filled with darker theories. Any moving name could be a mask. Any unfamiliar footprint could be a trap. Even the comfort of the Gryffindor common room didn't silence the buzzing thought: someone wants me dead.
Neville noticed the change most. "Harry… you're checking that map more than you sleep," he said nervously one night.
Harry's reply was curt: "Better tired than dead."
Hermione worried but didn't argue. She'd seen the scorch mark on Harry's basilisk-hide armor. The curse had been meant to kill.