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It was the middle of the night in their observatory hut, another winter night where they couldn't afford to wake Chrome downstairs. The fire had burned low, casting long shadows across the primitive bed of furs they'd piled together for warmth.
Senku sat with his back against the wall, bare-chested despite the cold, his breath fogging in the dim lamplight. Gen knelt beside him, still fully dressed in her layered winter clothes, one hand wrapped around his cock.
“Just do it already!” Senku hissed, frustration bleeding through his usually composed voice.
Gen quirked a brow, her face betraying nothing but amusement as Senku glared at her. His cock was thick in her grip, hot and rigid, the skin stretched taut. She stroked him slowly—more of a lingering tease than a real touch—and watched the way his jaw clenched, the way his breathing turned shallow.
He was about to reach down and take matters into his own hands. Gen saw it coming, that shameless scientist ready to jerk himself off right in front of her.
“No.” Gen slapped his hands away with her free one, firm and swift. “No ouching-tay yourself.”
He scowled, that familiar sharp frown cutting across his face. “There was never a rule in the first place.”
“Well, I'm making a rule now.”
“Why are you being such a tease?”
Because the face you make is so delicious to see.
Gen kept the thought to herself, letting the silence stretch. For someone who never lost his cool—who smirked in the face of danger like it was his bitch—watching Senku unravel was a gift she intended to savor.
His cheeks were flushed, a rare pink staining his usually pale skin. Sweat beaded at his temples despite the cold. His breathing came ragged, chest rising and falling fast. His cock twitched in her hand, desperate for more friction. He looked undone, and she loved it.
Gen smiled angelically. “What do you mean by that? I'm just touching you, like you wanted to experiment too.”
“Stop with the mentalist shit and touch me.”
“Hm, what if I won't blow you anymore?”
“I don't care anymore, just touch me—”
She didn't let him finish. With one swift movement, Gen sank down and took his cock into her mouth.
Senku choked.
The weight of him filled her mouth, hot and tasting of skin and salt. Gen remembered what her high school classmates had said in the locker room years ago—no teeth, unless you know what you're doing.
So she kept her teeth carefully hidden, pressed her lips tight around the shaft, and slid down slowly. She remembered the tips: use your tongue, flatten it along the underside, move with rhythm..
And Gen—Gen was a mentalist who had performed with a stick on fire and a sword pressed against her throat. She knew how to control her gag reflex, and had trained it through countless rehearsals. She could take him deep without choking, could let the head of his cock press against the back of her throat and keep going.
But before she could push that far, Senku came.
It spilled across her tongue in hot, sudden bursts, it’s bitter and thick. He groaned, a sound torn from somewhere deep in his chest, his hips jerking involuntarily as he emptied into her mouth. She held still, letting him ride out the wave, and when he was done, she swallowed every drop.
She pulled back slowly, licking her lips, and saw the mess she'd made of him. Senku's head was thrown back, eyes squeezed shut, mouth open. His chest heaved as his hands fisted in the furs. He looked wrecked
It was so hot.
Really, it was all part of the plan. Frustrate him, make him lose that cool composure, make him beg—then drop down and take him before he could brace himself. She'd wanted to see him fall apart, wanted to see that first blowjob hit him like a freight train.
To make Senku come too fast, almost in an embarrassing way.
And she'd gotten exactly that.
Senku opened his eyes and glared at her, almost pouting. Gen couldn't help the laugh that escaped her.
“You really did that on purpose,” Senku sighed, his voice still rough.
“Orry-say.” Gen smiled, not apologetic at all. “I promise you next time, I'll give you the best blowjob.”
He waved a dismissive hand. “That's a promise. Although… it was pretty great to me.”
“I'm glad.”
They both knew that was the truth. For a first blowjob, it had been brilliant—not because of any technical perfection, but because Senku hadn't had time to sort himself out, hadn't had the chance to steel his nerves or prepare mentally.
Next time, they'd both be ready. Next time, he'd last longer. But this had been exactly what they needed: a raw, unscripted first taste.
“Also,” Senku added, a note of genuine surprise in his voice, “you didn't have to swallow my cum.”
Gen shrugged. “Just think of it as part of the experiments. It's not like we'll get an STD.”
“You and I both know, despite us having no experience whatsoever, STDs can transfer without any sexual intercourse—”
“Ugh, just trust that I don't mind you cumming in my mouth, so shut it, Senku-chan.”
“Fine.” He grumbled, but the corner of his mouth twitched. “Then it's fine if I do the same, right?”
Gen blinked.
Senku grinned, that manic, brilliant grin she knew too well. “Now…” He sat up, reaching for the waistband. “Let me eat you out.”
Gen swallowed the sudden tightness in her throat, but she nodded. After all, that had been their agreement in this experimentation.
She slowly removed her clothes, starting with the simple wrap of her primitive yukata, letting it fall open. The cold air kissed her skin, raising goosebumps, but she didn't bother keeping the fabric around her. She slid off the undergarment beneath it—a simple cloth wrap—and let both layers pool beside her on the furs.
As Gen laid her body down, Senku moved between her legs, settling on his stomach. The earlier blowjob had already made her wet—she could feel the slickness between her thighs, the way her body anticipated his touch.
"It seems you are ready, mentalist." Senku smirked, that familiar smug tilt to his mouth.
Gen rolled her eyes. "Shut up."
"You're the one who's already dripping."
"And you're the one who came in under a inute-may. We all have our things."
Senku's smirk faltered slightly, but the amusement in his eyes remained. "Fair enough."
He didn't waste more time on words. His fingers found her clit first—not her entrance, not teasing at her opening, just her clit. He touched it experimentally, circling it with the pad of his finger, watching her reaction the same way he'd watch a chemical reaction in a flask.
Gen groaned, her hips twitching involuntarily. The touch was direct, sending sparks straight through her nerves.
"Interesting," Senku murmured, more to himself than to her. He changed the pressure—lighter, then firmer. He circled clockwise, then counterclockwise. He pressed down and moved side to side.
"Are you cataloguing data right now?" Gen asked through gritted teeth.
"Yes. Quiet, I'm experimenting."
She wanted to argue, but his thumb pressed against her clit in a slow, deliberate circle, and the words died in her throat.
He glanced up briefly, and Gen looked away, feeling heat rush to her cheeks. Senku cleared his throat, also looking away for a moment before returning his attention between her legs.
His fingers traced downward, circling her entrance without pushing in. The tip of his finger collected some of her wetness, spreading it back up to her clit. He did it again, a slow circuit—her clit, then down to her entrance, then back up. Each pass made her breath hitch.
"Is this evenge-ray from earlier?" Gen grumbled. "Just hurry up and stop treating my vagina like this."
"It would be tempting to tease you, but—" Senku licked his lips, and his voice dropped lower. "—your cunt looks really inviting right now."
Before Gen could ask what he meant by that, Senku dove down and put his tongue on her.
The first touch was clumsy—his tongue too flat and broad, pressing against her clit without enough precision. Gen gasped, but it wasn't quite right. Senku paused, lifted his head, and frowned.
"Too much surface area?"
"I don't know, you're the genius."
He tried again, this time with the tip of his tongue, pointed and precise, tracing small circles around her clit. Gen's breath caught. That was better. That was good.
He seemed to notice her reaction—the sharp inhale, the way her fingers curled into the furs—and repeated the motion.
Then he varied it: flicks, long slow drags, quick flutters. He wasn't reading a manual; he was reading her body, adjusting based on every twitch and gasp.
Of course, Gen thought hazily. Of course he'd be a genius at this too.
But then he sucked her clit gently into his mouth, and her mind went blank.
She couldn't think. Couldn't form words. His tongue moved against her in a rhythm she couldn't predict, alternating between broad strokes and sharp points of pressure. He hummed against her—a low, contemplative sound—and the vibration sent a shock through her entire body.
She bit her lip hard, trying to stay quiet. Chrome was just downstairs. One loud moan and they'd have to explain everything. So she pressed her fist against her mouth, breathing hard through her nose as Senku worked her closer to the edge.
He wasn't looking at her. His eyes were focused entirely on what he was doing, studying her like a particularly fascinating specimen. But Gen felt the curve of his lips against her skin. He was smirking. Even without seeing his face, she knew.
And then she came.
It hit her in a wave, starting from where his tongue pressed against her clit and radiating outward. Her thighs clenched around his head, her back arched off the furs, and a strangled sound escaped through her fingers. Pleasure pulsed through her in hot, rhythmic waves, each one making her muscles flutter and release.
She lay there, panting, staring at the ceiling as the world slowly came back into focus. Her limbs felt loose, heavy, like they'd been unstrung.
As Gen regained her bearings, she looked down at where Senku was and blinked in surprise.
He had tilted his head, still positioned between her legs, and was staring at where she'd cum. His expression was satisfied but also curious—the same look he got when an experiment yielded unexpected results.
Before Gen could ask what was wrong, Senku dove back in.
But this time, he used his fingers.
"Wait, Senku-chan—oh!"
She clapped both hands over her mouth.
His tongue found her clit again, but now his fingers joined in—not pushing inside her, but spreading her open, pressing against the hood of her clit, rolling the sensitive bundle of nerves between his fingertips while his tongue flicked over the tip.
It was too much. She had just come. Every touch felt amplified, too sharp, too intense. Her hips tried to squirm away, but Senku's other arm hooked around her thigh, holding her in place.
"Sensitive," he murmured against her, and there was a note of discovery in his voice.
He changed technique. Slower, softer and lighter circles that barely brushed against her oversensitive clit.
Senku let his breath warm her before his tongue made contact, teased the edges without committing to the center. It should have been frustrating—it was frustrating—but it also built a different kind of pressure.
Gen's muffled screams vibrated against her palms. Her eyes were watering. Every nerve in her body felt like it was singing.
When she came again, it was almost silent—a full-body shudder, her thighs trembling, her vision going white at the edges. She couldn't breathe or think.
She stared at the ceiling, chest heaving, completely spent. Her body felt like it had been wrung out and hung to dry.
"Was that good?" Senku asked, tilting his head. There was something almost vulnerable in his voice, masked by casual curiosity.
Gen almost scoffed, but she was too out of breath. "It is good—"
She didn't finish. Senku moved before she could, easily manhandling her exhausted body. He flipped her onto her stomach, grabbed her hips, and pulled her ass up. Before Gen could process what was happening, he had his tongue against her entrance again.
She screamed into her pillow, muffling the sound as best she could. He was licking into her now, his tongue pushing past her folds, tasting her from the inside. His hands gripped her ass, spreading her open, and he buried his face deeper.
Gen's body didn't know what to do. It was too soon, too much, too overwhelming—but it also felt impossibly good. Her fingers clawed at the furs. Her hips bucked against his mouth involuntarily.
She came again, a third time, with a broken sob pressed into the fabric of the pillow.
When Senku finally pulled back, Gen scrambled away from him.
She sat up immediately, despite her body screaming at her to lie down, and her thigh was trembling, and glared at him.
He raised both hands in surrender, looking genuinely sheepish.
"I just got carried away," Senku said, a nervous chuckle escaping him. "I don't know. There's something… stimulating about the response. The way your body reacts. It's like—there's a feedback loop. The more you respond, the more I want to push further. It's almost addictive in a neurological sense."
"Oral fixation," Gen said, her voice hoarse.
"Huh?"
"You have an oral fixation, Senku-chan."
Senku blinked at her, then looked contemplative. "Oral fixation… based on the words alone… maybe you're right."
"Give me a arning-way next time, will you?"
"Yeah… you're right about that."
"Why do I hear a 'but' in there?"
"Well, scientifically speaking, women can have multiple orgasms without a refractory period. It's inefficient to stop when the body is clearly capable of more. If you consider it from a purely evolutionary standpoint, there's no reason to limit—"
Gen smacked him on the head.
"Ow."
"You deserved that."
"... I know."
She was about to say something else—maybe a sharp remark about his lack of self-control—when her eyes drifted downward. Senku's cock had gotten hard again. It stood upright, flushed and gleaming slightly in the low firelight, as he kept talking about evolutionary biology like nothing was happening.
After what happened earlier, Gen wanted to do an absolutely amazing job.
So without thinking, she reached out and wrapped her fingers around his cock.
Senku stopped mid-sentence.
Gen watched his throat work as he swallowed. His composure flickered, just for a moment.
"Revenge?" he asked, his voice slightly strained.
"Omething-say like that."
Gen gave Senku another chance to back out, her smirk already curving at the corner of her mouth. He said nothing—just watched her with that familiar, calculating gaze, one elbow propped on the furs as if he were analyzing a particularly interesting reaction in one of his bamboo flasks. That was all the confirmation she needed.
"Feel free to use my hair, Senku-chan," she said, her voice low and teasing, "and don't come fast this time."
Before he could respond, she dropped her head toward his cock.
Senku was already half-hard from the earlier play. Gen let her breath ghost over the tip first with a deliberate tease—then dragged the flat of her tongue from the base to the crown in one long, wet stroke. He tasted salty and clean, a hint of sweat from their earlier wrestling on the fur bed. She repeated the motion, circling her tongue around the head, dipping into the slit just enough to make his hips twitch.
She took him into her mouth, just the head at first, sucking lightly while her fingers wrapped around the shaft and pumped in a lazy counter-rhythm. Her other hand rested on his thigh, feeling the muscle jump under her palm.
Gen worked her way down inch by inch, relaxing her throat. The familiar pressure built at the back of her tongue, and she paused—a small, controlled gag—then eased off, letting the muscles adjust before pushing deeper again.
Then she found that perfect edge, the tip pressing against the entrance of her throat, and held there, letting her tongue work the underside of his cock.
Senku's breathing had gone ragged. His fingers curled into the furs at first, a white-knuckled grip that told her he was fighting the urge to thrust. She hummed around him, a low vibration, and his whole body shuddered.
One hand moved, trembling slightly, and threaded into her hair. He hesitated, grip loose, as if asking for permission. Gen looked up at him through her lashes, her eyes meeting his. That seemed to break something.
He grabbed a fistful of her hair, firm but not painful, and began to move her head—slowly at first, guiding the pace. Gen let him take control, closing her eyes the better to feel it.
Every time he pushed her down, his cock slid deeper into her throat, and she hollowed her cheeks, increasing suction, breathing through her nose in a steady rhythm. Senku's grip tightened. A low moan escaped his lips, raw and unguarded. His hand fisted in her hair, and he started moving her faster, driving himself deeper with each thrust.
Gen watched him through half-lidded eyes: his jaw was clenched, a flush crept up his neck, and his chest rose and fell in quick, shallow breaths. He looked undone—that ever-calm scientist reduced to a mess of pleasure.
She loved it.
The way his eyelids drooped, the way his hips bucked involuntarily, the way his mouth fell open on a gasping moan when she swallowed around him. She increased the pressure of her lips, the speed of her hand pumping the base in sync with her mouth, and Senku's rhythm turned jerky. His whole body tensed.
"Gen—" His voice cracked. His hand yanked her hair, holding her down as he came, hot and thick down her throat.
She swallowed, milking him through the aftershocks, until he finally stilled, panting.
She pulled off slowly, there was some cum she hadn’t managed to catch around her mouth. They both lay there, gasping, the air thick and warm between them.
Senku sat up, still catching his breath. Without a word, he tugged off the thin linen undershirt he wore—the inner layer beneath his lab coat—and, with surprising gentleness, wiped his own cooling release from Gen’s chin and cheek.
The fabric came away damp. He folded the shirt, the cleanest part outward, and then knelt between her legs. Gen watched, curious, as he carefully dabbed at the slickness there, cleaning her with a tenderness that felt almost ridiculous after what they'd just done.
"Did I miss a spot?" she teased, voice rough.
"Just being thorough," he muttered, but a faint smile tugged at his lips.
He tossed the shirt aside and sat back on his heels, crossing his arms. The flush on his cheeks hadn't faded.
Gen stretched languidly, propping herself up on her elbows. "So how was the experiment, Senku-chan?"
A slow smirk spread across his face, the old confidence seeping back into his posture. His hand came up to rest on his chin, index finger tapping lightly in that familiar, calculating gesture.
"Ten billion percent perfect."
Gen snorted, flopping back onto the furs. "High raise-pay from the science guy. I'll take it."
"You should." He lay down beside her, staring up at the thatched ceiling of the observatory hut.
Senku’s hand found hers in the dark, but it’s Gen who intertwined their fingers without a word. Through the gap in the roof, a sliver of moon cast pale light across their tangled bodies.
She squeezed once, then let herself smile.