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The Professor's Potion

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A crack AU. Two male best friends in college, Hyukjae and nerdy Donghae. Hyukjae was involved in an experiment (by professor Kim Heechul) where he turns into a voluptuous lady after taking the professor concoction. He could turn to male only after someone comes in him. So he could only ask his best friend Donghae to turn him back into a male. Donghae who had always love Hyukjae (even as a male) agreed willingly.

Chapter 10: Claiming What's His

The chill came first, seeping up from the tile floor through their knees. Then the stiffness. The glorious, molten heat between them cooled into a sticky, uncomfortable mess. Donghae was the first to shift, his softening length slipping free, followed by a faint, wet sound that made Hyukjae flinch.

“We should…” Donghae’s voice was a rough scrape against the quiet. “We should clean up.”

Hyukjae just nodded, his forehead still pressed to the wall. He felt scraped out, hollowed and filled simultaneously, a paradox of sensation he couldn’t begin to name. Donghae’s hands left his hips, and the loss of contact felt like falling.

The shower came on again, a hiss of hot water. Donghae guided him under the spray, gentle now, all the earlier ferocity spent. The water sluiced away the evidence, swirling it down the drain. They didn’t speak. Donghae soaped a washcloth and cleaned them both with a clinical tenderness that made Hyukjae’s throat tighten. It was a caretaker’s touch, intimate in a completely different way.

He was towelled dry with the same quiet efficiency, the rough fabric chasing the last of the goosebumps from his skin. Donghae wrapped him in a clean towel and then, without a word, lifted him—an arm under his knees, another behind his back. Hyukjae didn’t protest. He let his head loll against Donghae’s shoulder, exhaustion a lead weight in his bones.

The short walk to the bedroom felt like a journey between worlds. The chaos of the main room—the overturned coffee table, the discarded clothes—was a monument to what had happened. Donghae stepped over it all and shouldered the bedroom door open.

The bed was a disaster of rumpled comforters, but Donghae laid him down in the centre of it anyway, peeling back the damp towel. The air was cool on his bare skin. Donghae disappeared for a moment, returning with a glass of water.

“Drink.”

Hyukjae obeyed, the water shockingly cold. He drained the glass. Donghae set it aside, then slid into bed beside him, not touching him at first. They lay on their backs, staring at the shadows on the ceiling, listening to the steady drip of the bathroom tap.

“So,” Hyukjae said finally, the word too loud.

“So.”

Another long silence. Hyukjae’s mind, previously blissfully numb, began to spin. The questions were there, a swarm of them, but one buzzed louder than the rest.

“Did you mean it?” he asked, his voice small. “What you said. About… wanting me. The mind, the laugh. All of it.”

Donghae turned his head on the pillow. In the dim light from the cracked door, his eyes were black pools. “Every word.”

“Even when I was… her?”

“Especially then,” Donghae said. “Because it was still you, and it was killing me. Watching you walk around in that skin, knowing you hated it, knowing it was temporary… and wanting you so much I couldn’t breathe.”

Hyukjae rolled onto his side, facing him. “And now?” He gestured vaguely at his own body—the flat chest, the narrow hips, the familiar cock lying soft against his thigh. “Now that I’m… back?”

A slow, heartbreaking smile touched Donghae’s lips. “Now I’ve had you both ways. And I want you all the ways.” He reached out, his fingers tracing the line of Hyukjae’s jaw. “This is the face I fell for. This is the body I’ve watched dance for years. It’s familiar. It’s home.”

The words unspooled something tight and terrified in Hyukjae’s chest. He moved then, closing the distance between them, burying his face in the curve of Donghae’s neck. He breathed in his scent—soap, sleep, Donghae. “I’m scared,” he whispered, the confession muffled against skin.

“Of what?”

“That it was the freakshow that made you finally look at me. That without it… we go back to normal.”

Donghae’s arms came around him, strong and certain. “There is no back. There’s only forward from here.”

He kissed Hyukjae’s temple, his cheek, the corner of his mouth. It was a soft question. Hyukjae answered by turning his head and catching Donghae’s lips with his own. This kiss was different again—not the desperate claim of the shower, nor the gentle exploration before it. This was deep, languid, a slow tasting. It was familiarity discovering a new dimension.

Hyukjae shifted, throwing a leg over Donghae’s hips, settling over him. The thin blanket pooled around their waists. He could feel Donghae stirring against him, hardening once more. A flush of power, dizzying and new, rushed through him. He broke the kiss, looking down at Donghae’s flushed face.

“Say it again,” Hyukjae murmured.

“What?”

“That you want me. Like this.”

“I want you,” Donghae breathed, his hands coming up to grip Hyukjae’s thighs. “God, Hyukjae, I want you so much.”

A slow, mischievous smile—the first genuine one since the change—spread across Hyukjae’s face. It felt good on his own lips. He sat up, straddling Donghae’s waist, looking down the length of their bodies. He took himself in hand, giving a few lazy strokes, watching Donghae’s eyes darken with hunger.

“You liked it,” Hyukjae said, his voice dropping, taking on a teasing, sinful edge. “When I was her. You liked eating her out. You fucked her like you wanted to die in there.”

Donghae’s breath hitched. “Yes.”

“You remember how she tasted?”

“Yes.”

Hyukjae leaned forward, bracing his hands on Donghae’s chest. He rocked his hips, his hardening cock dragging through the trail of hair on Donghae’s stomach. “You want to taste me now?”

Donghae’s answer was a groan, his head tipping back against the pillow. His hands tightened on Hyukjae’s hips, urging him up.

With a fluid, deliberate move, Hyukjae shifted backwards, sliding up Donghae’s body until he was kneeling over his face. He looked down between his own legs, at Donghae staring up at him, eyes wide, lips parted. The vulnerability of the position was acute, a surrender of control that sent a thrill through him.

“Go on,” Hyukjae said, his voice a low command. He reached behind himself, spreading his cheeks, offering himself. “Taste the difference.”

Donghae didn’t hesitate. His hands came up, gripping the backs of Hyukjae’s thighs, pulling him down. His tongue pressed against Hyukjae’s entrance in one broad, wet stroke.

Hyukjae’s head fell back with a choked gasp. The sensation was immediately, overwhelmingly intense. Donghae’s tongue was warm, relentlessly probing, licking into him with a shameless hunger that bordered on worship. It was messy, obscene, the sounds loud in the quiet room. Donghae ate him out like a man starving, his tongue pushing deep, then flattening against his perineum, then circling the tight ring of muscle before plunging back in.

“Fuck… yes… just like that,” Hyukjae moaned, his fingers tangling in his own hair. He rocked his hips, grinding down against Donghae’s mouth, chasing the building pressure. The wet heat, the scratch of stubble on his sensitive skin, the absolute filth of it—it was better than before. It was real. “Your tongue… so deep… you fucking love this, don’t you? Love having me here.”

Donghae’s response was a muffled, fervent moan against him, the vibration shooting straight up Hyukjae’s spine. One of Donghae’s hands left his thigh, fingers slick with saliva probing forward to fondle Hyukjae’s balls, then wrap around his cock. The dual sensation was too much. Hyukjae cried out, his thighs beginning to tremble.

He was hurtling towards the edge, too fast, the coil in his gut winding tight. He wanted to come like this, come all over Donghae’s face, mark him. But a sharper, deeper need took over.

“Stop,” he gasped, pushing himself up. “Stop, I need… I need you inside me. Now.”

Donghae released him, his mouth and chin glistening. His eyes were wild. Hyukjae scrambled off him, turning onto his hands and knees on the mattress. He looked back over his shoulder, breathing heavily. “Don’t be gentle.”

Donghae moved behind him in a flash. He positioned himself, the blunt head of his cock nudging against Hyukjae’s loosened, wet entrance. He didn’t push. Not yet.

“Look at me,” Donghae said, his voice guttural.

Hyukjae twisted, meeting his gaze over his shoulder.

“This is you,” Donghae said, holding the eye contact. “This is me. This is us.”

Then he thrust, hard and deep, burying himself in one smooth, brutal stroke.

The cry that tore from Hyukjae was one of pure satisfaction. The stretch, the burn, the profound feeling of being filled—it was homecoming. Donghae set a punishing rhythm immediately, each slam of his hips driving Hyukjae forward on the mattress. He gripped Hyukjae’s waist, his fingers digging in, surely leaving bruises.

The bed creaked in protest. The headboard knocked against the wall in a steady, frantic tattoo. Hyukjae pushed back against him, meeting every thrust, their bodies colliding with a wet, slapping music.

“Yes! Fuck! Right there!” Hyukjae chanted, his voice breaking on each syllable. “Harder, you bastard, fucking own me—”

Donghae leaned over him, his chest plastered to Hyukjae’s back, his mouth at his ear. “Mine,” he growled, the word a promise and a threat with each piston of his hips. “You’ve always been mine.”

The claim shattered the last of Hyukjae’s control. He came untouched, his orgasm ripping through him with a silent, breathless violence, his channel clamping down on Donghae in rhythmic, milking spasms. The intensity pulled Donghae over with him. He drove in one last, deep time, holding himself there as he emptied, his own shout lost in the sweat-damp skin of Hyukjae’s shoulder.

They collapsed together, a heap of trembling limbs and spent breath. Donghae slipped out, rolling onto his back, pulling Hyukjae against him. They lay in the wreckage of the bedding, the only sound their ragged breathing slowly returning to normal.

Hyukjae’s mind was quiet. The ghosts were gone, silenced by the sheer, undeniable physical truth of what had just happened. He traced idle patterns on Donghae’s chest.

“We have to see Heechul,” he said into the dark.

Donghae’s hand stilled on his back. “Tomorrow.”

“Tomorrow,” Hyukjae agreed. But for tonight, there was only this: the warmth of the body beside him, the steady heartbeat under his ear, and the quiet certainty that whatever came next, they would face it from the same side of a door that could never be closed again.

But for tonight, there was only this: the warmth of the body beside him, the steady heartbeat under his ear, and the quiet certainty that whatever came next, they would face it from the same side of a door that could never be closed again.

Donghae woke to the familiar weight of Hyukjae’s arm slung over his ribs, a comforting anchor. Grey dawn light bled around the edges of the blinds. He inhaled, expecting the scent of Hyukjae’s cheap shampoo and sleep-warmed skin. The fragrance that met him was wrong.

It was softer, floral, laced with something unnervingly sweet. It was her perfume. The one from the drugstore, the one she’d sprayed in a fit of frustration three days ago.

A cold spike drove through his gut.

He turned his head slowly on the pillow.

The face resting there was Hyukjae’s, but not. The sharp jawline had softened into a gentle curve. The brows were thinner, the eyelashes absurdly long and dark against pale cheeks. His short, messy hair was gone, replaced by a tangled spill of dark silk that fanned across Donghae’s shoulder.

The weight on his chest was different. Fuller.Softer.

Donghae lay perfectly still, not breathing. The certainty of last night curdled into a sharp, metallic fear. His movement, or his sudden stillness, must have disturbed her. Hyukjae’s eyes—her eyes now, a shade lighter, a fraction larger—fluttered open.

Confusion swam in them for a moment, the usual disorientation of waking. Then she felt it. The subtle, impossible shift of mass. The drag of long hair on her neck. Her hand lifted from Donghae’s chest, her own hand, but slender-fingered, nails neatly shaped. She stared at it as if it belonged to a stranger.

A tiny, broken sound escaped her. She shoved the covers back, her movements frantic and uncoordinated, and looked down at her own body. The flat plane of a chest was gone. In its place were full, heavy breasts, the nipples tight and pink in the cool air. The sheet pooled at a waist that dipped in before flaring out to hips that were undeniably, frustratingly female.

“No.” The word was a whisper, torn and raw. “No, no, no, not again, please, no—”

She scrambled from the bed, stumbling on legs that felt alien, and half-ran, half-fell towards the mirror on the back of the bedroom door. She gripped the edges of the frame, her knuckles white. The reflection staring back, wide-eyed and horrified, was the woman from the storm. The experiment.The prison.

A high, thin whine built in her throat. It cracked open into a sob.

“Why?” she screamed at her reflection, the sound ragged and agonized. “Fucking why? It was over! I was back!”

She smashed her fist into the glass. It shuddered in the frame but didn’t break. She hit it again, a weak, fleshy thud. Then her legs gave out. She slid down the door, a crumpled heap of shaking limbs and silken hair, her back to the mirror she couldn’t bear to face. Great, heaving sobs wracked her. They weren’t graceful or quiet. They were ugly, gasping things that stole her breath and left her mouth wet and open.

She cried for the self she’d just reclaimed, now ripped away. She cried for the confusion, the profound violation of her own physical reality. She cried because she had just found solid ground with Donghae, only to have the earth turn to liquid beneath her again.

Donghae was already there. He knelt on the floor in front of her, ignoring his own nakedness, the cold of the boards against his knees. He didn’t try to pull her hands from her face. He didn’t tell her to stop.

He just gathered her. He pulled her into his lap, her back against his chest, her body shaking violently against his. He wrapped his arms around her, one across her collarbone, the other around her waist, holding her together as she fell apart. Her sobs were muffled against his forearm.

“I can’t,” she choked out, the words garbled. “I can’t do this again. I can’t be this… this thing.”

“You’re not a thing,” Donghae said into her hair, his voice low and steady, a bedrock against her tempest. “You’re Hyukjae.”

“Look at me!” she wailed, gesturing wildly at her own body, at the curves his arms were wrapped around. “This isn’t me! It’s a fucking costume! A sick joke!”

“I know.” He held her tighter. “I know it feels that way. It’s not fair. It’s a nightmare.” He nuzzled into the space behind her ear, breathing in that wrong, sweet perfume that was now just her. “But listen to me. Right now. Listen.”

She shuddered, her cries subsiding into harsh, wet hiccups.

“I love you.” The words were clear, absolute. “Not the body.You. The smart-ass who steals my fries. The idiot who laughs too loud at his own jokes. The man who shoved me against the shower wall last night.”

A fresh sob escaped her, but this one sounded different. Wounded, but listening.

“I love the woman who kissed me in the rain,” he continued, his lips moving against her temple. “The one who was brave enough to ask for what she wanted, even when it terrified her. I love that version, too. She’s still you. All of it is you.”

He turned her gently in his arms, forcing her to look at him. Her face was a mess—swollen eyes, blotchy skin, strands of hair stuck to her wet cheeks. He cradled her face in his hands, his thumbs wiping uselessly at the tears.

“This body?” he said, his gaze holding hers fiercely. “It’s beautiful. It’s yours. And I will worship every inch of it, just like I did the one before, because it houses you. We will fix this. We will find Heechul and burn his lab down. But until then, you are not alone. You are not a freak. You are mine. And I am yours. However you come.”

The fight seemed to drain out of her. The violent trembling eased into smaller, sporadic shivers. She searched his eyes, looking for pity, for hesitation, for a lie. She found only a raw, terrifying honesty that mirrored the one she’d seen last night.

She leaned forward, resting her forehead against his. Their breath mingled, hot and damp. “I’m scared,” she whispered.

“I know.”

“I hated it before.”

“I know that, too.”

“But last night…” she trailed off, the memory of their coupling in this very room, in his male body, flashing between them.

“Last night was real,” Donghae finished for her. “This is real. The want is the same. The need is the same.” His hands slid from her face, down the sides of her neck, over the new, shocking softness of her shoulders. He traced the outer curve of her breast with the back of his knuckles, a feather-light touch. “Let me show you.”

She didn’t nod. She just closed her eyes, a tear squeezing from beneath her lid, and let her head fall back. It was surrender. It was permission.

Donghae lowered his mouth to her collarbone, kissing the salt from her skin. He followed the line of her breast, his tongue painting a slow, wet path towards her nipple. When he took the peak into his mouth, suckling gently, she gasped. The sensation was wildly different, a direct, electric line to her core that her old body had never known.

His hands were everywhere, learning this new geography. They cupped the heavy weight of her breasts, thumbs brushing the stiffening peaks. They slid down the dramatic inward curve of her waist, spanning it possessively. They gripped the lush flare of her hips, fingers digging into the soft flesh.

“You feel that?” he murmured against her skin, his mouth moving lower, across the quivering plane of her stomach. “That’s all you. Your nerves. Your pleasure.”

He parted her thighs, his hands pushing them wide. He knelt between them on the hard floor. The morning light caught the slick evidence of her arousal glistening on the soft, dark curls. The sight of her, spread open and wet for him in this new form, made his breath catch. He leaned in, his breath hot against her.

“Donghae…” Her voice was a shaky whisper.

“Shhh. I’ve got you.”

He didn’t use his tongue. Not at first. He pressed his mouth to her in a deep, open kiss, his lips sealing over her entire sex. He sucked, pulling the swollen flesh into the heat of his mouth.

Her back arched off the door, a sharp cry tearing from her. Her hands flew to his hair, fisting in the dark strands. He licked then, a broad, flat stroke from her opening to her clit, circling the hard little nub before plunging his tongue deep inside her.

The taste was different. Muskier, sharper, uniquely her. He drank her down, his tongue fucking her in shallow, relentless thrusts. He added a finger, sliding it easily into her dripping cunt, crooking it to find the spot that made her scream.“There! Oh fuck, right there!” she begged, her hips rolling uncontrollably against his face. “Don’t stop, please don’t stop, I’m gonna—“

He added a second finger, stretching her, his mouth never leaving her clit. He sucked it, flicked it with his tongue, worshipped it. Her thighs clamped around his head, her body bowing into a tight, tense arc. Her orgasm hit like a seizure, a silent, breathless implosion before the sound followed—a raw, ragged wail that echoed in the small room. Her cunt clenched rhythmically around his fingers, flooding his hand with wet heat.

He didn’t let her come down. He rose over her, his own need a painful throb. He gripped his cock, guiding the head through her slick folds, notching it at her entrance. Her eyes, dazed and satiated, focused on his. She reached between them, her delicate hand wrapping around his length, positioning him.

“Do it,” she breathed, her voice wrecked. “Fuck me. In this body. Make it mine.”

He pushed in. The resistance was different. A tighter, velvety grip that gave way all at once, swallowing him whole in a single, smooth slide. She was so deep, so incredibly wet and hot. She gasped, her mouth forming a perfect ‘O’ of shock and pleasure.

He began to move. Slow, at first, each withdrawal almost complete, each thrust a deliberate, full re-possession. The new angle, the way her hips cradled him, the feel of her breasts pressed against his chest—it was a revelation. It was the same soul, wrapped in a different, devastatingly sensitive package.

“You feel that?” he grunted, pounding into her harder now, her body sliding on the polished floor with each drive. “That’s me. In you. Same as last night. Same as always.”

“Yes!” she cried, her nails scoring his back. “God, yes, it’s you, it’s you, it’s you—”

Her legs hooked around his waist, pulling him deeper. The slap of skin grew faster, more frantic. He could feel another climax building in her, her inner muscles fluttering around him. He reached between them, his thumb finding her clit, rubbing rough, tight circles.

It tipped her over. She came again with a choked scream, her cunt milking him violently. The rhythmic convulsions pulled his own orgasm from him. He thrust once, twice more, burying himself to the hilt as he emptied into her with a guttural roar, painting her insides with his release.

They collapsed together on the floor, a tangled, sweating heap. He was still inside her, softening. She was limp, her face buried in his neck. The silence was thick, broken only by their slowing breaths.

After a long while, she spoke, her voice husky. “We still have to burn Heechul’s lab down.”

A laugh rumbled in Donghae’s chest. “We will.”

She tilted her head back, looking at him. The profound grief from minutes before was still there, in the depths of her eyes, but it was now edged with a hard, glittering resolve. And something else. A fragile, newborn acceptance.

“But first,” she said, her fingers tracing the line of his jaw. “Coffee. I need fucking coffee.”

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