Chapter 1: Tidepool Enclosure
The Tidepool Research Station hummed. It was a low, constant vibration beneath the soles of Izuku’s sneakers, a symphony of pumps, filters, and computers married to the distant, rhythmic crash of waves against the cliff below. Salt air, sharp and clean, mingled with the faint ozone scent of electronics. Maps of ocean trenches were pinned to every available wall, their depths marked in careful, handwritten notes.
Izuku leaned over the main observation tank, his freckled face bathed in the cool blue glow. His green curls fell into his eyes as he scribbled on a waterproof notepad. “Remarkable epidermal regeneration,” he muttered to himself. “The scarring from the netting is nearly gone.”
In the water, Katsuki Bakugou glared back, his crimson eyes slitted. He floated, arms crossed over his chest, his powerful lionfish tail—a stunning tapestry of vibrant orange, white, and red stripes—giving an occasional, irritable flick. The fins along his forearms flared slightly. “Stop talking about me like I’m a specimen, nerd.”
“You are a specimen,” Izuku said, not looking up. “A fascinating one. Your venom sacs alone could—”
“Could melt your face off. Yeah, yeah. You’ve said.” Katsuki propelled himself forward in a sudden, smooth motion, planting his hands on the thick acrylic. The webbing between his fingers pressed white against the surface. His upper body, sculpted and lean, was a topography of muscle. “You’re boring me. Do something interesting.”
Izuku finally met his gaze, a faint smile touching his lips. “What would you consider ‘interesting,’ Kacchan?”
The old childhood nickname, deployed with deliberate gentleness, always had an effect. Katsuki’s nostrils flared. A low, guttural sound, not quite human, rumbled in his chest. “Don’t call me that.” But the threat was hollow, a habit. He pushed off from the wall, twisting in a slow, lazy circle, displaying himself. The spines along his back and tail gleamed under the lights. “Anything. Fight me. Feed me. Get in the water.”
From the shadowed mouth of a large artificial cave structure deep in the enclosure, another pair of eyes watched. Sophia remained perfectly still, her dark, iridescent scales blending with the rock. Her long, thick octopus-hybrid tail, dotted with sensitive suckers, was coiled beneath her. One of her spiky tendrils idly traced patterns in the sand.
She watched the way Izuku’s whole posture leaned toward the tank, absorbed. She saw the way Katsuki’s performative anger was just a shell, something he shed when he thought no one was looking. Right now, he was looking at Izuku like the human held the moon in his hands.
A heavy, dull ache settled in Sophia’s chest. She wasn’t like them. Izuku was all soft curiosity and bright light. Katsuki was fire and sharp edges and brilliant color. She was… dark. A deep-sea thing. Chubby where Katsuki was cut, quiet where he was loud, her body built for the crushing silence of the abyss, not for this sun-dappled tank or the attention of a brilliant, green-haired man.
She saw Katsuki say something, his expression shifting from a scowl to a sharp, challenging grin. Izuku laughed, a warm, surprised sound that echoed faintly in the humid room. The sound was a physical blow to Sophia.
She wasn’t their type. Of course she wasn’t.
Before the feeling could fully solidify into a sob, she pushed off from the cave floor. Her powerful tail thrust sent her gliding silently into the densest part of the kelp forest at the tank’s far end, her dark form dissolving into the swaying fronds. She wanted to disappear, to sink into the cool, familiar embrace of the deepest, darkest part of her world.
Izuku ran a hand through his hair, flustered. “Get in the water? Kacchan, it’s not protocol for non-essential—”
“Fuck your protocol.” Katsuki was at the surface now, his upper body resting on the submerged platform. Water sluiced down the defined planes of his chest. “You study me. So study. Come feel how the musculature works. Firsthand.” His grin was all teeth, predatory and inviting.
Izuku’s heart hammered against his ribs. This was unprofessional, reckless, a dozen things he’d written memos about. But Katsuki’s eyes were blazing, and the challenge in them was a magnet. He found himself kicking off his sneakers, peeling off his socks. “Just… just for a minute. For research.”
“For research,” Katsuki echoed, the words a dripping, sarcastic purr.
The water was cooler than Izuku expected, shocking his skin as he lowered himself in. He wore only his simple lab pants, now soaked and clinging. Katsuki watched him, motionless, like a lionfish waiting for a shrimp to wander into range.
“Well?” Katsuki growled. “Come here, then.”
Izuku paddled closer. As soon as he was within reach, Katsuki’s hand shot out, not with violence, but with startling possessiveness. His webbed fingers wrapped around Izuku’s bicep, pulling him flush against his chest. Izuku gasped. The feel of Katsuki’s skin was unlike anything—smooth, but with a subtle, textured pattern like the finest sandpaper. He was radiating heat.
“See?” Katsuki’s voice was a hot whisper against Izuku’s ear. His other hand slid down Izuku’s side, over his hip. “Built for thrust. For speed. Not like some sluggish bottom-feeder.” The insult for Sophia was casual, pointed.
“Don’t call her that,” Izuku murmured, but his protest was weak, his body arching into the touch. His own hands came up, tentatively tracing the line where skin met the vibrant fin along Katsuki’s shoulder. He felt the powerful muscle beneath twitch.
Katsuki made a satisfied, rough sound. “You like it.” It wasn’t a question. He shifted, his tail coiling around Izuku’s legs, not tight enough to trap, but enough to feel its immense strength. The touch of the spines was a deliberate, dangerous caress. “You’ve been staring for weeks. All those notes. You just wanted to get your hands on me.”
Izuku’s breath hitched. Denial died in his throat. He had wanted this. The intellectual fascination had curdled, heated, become something else entirely. He turned his head, his lips accidentally brushing the salty skin of Katsuki’s collarbone.
That was all the invitation Katsuki needed.
He claimed Izuku’s mouth with a furious, consuming hunger. It wasn’t a kiss; it was a conquest. All teeth and demanding tongue and guttural noise. Izuku whimpered, his hands tangling in Katsuki’s spiky hair, holding on as the world dissolved into sensation—the heat of the mouth on his, the slick coil of the tail around his thighs, the abrasive slide of skin against his own.
Katsuki broke the kiss, breathing ragged. “Mine,” he snarled, the word raw and primal. His hands went to the waistband of Izuku’s pants, claws snagging on the fabric. A quick, brutal yank and the material tore. Izuku cried out, partly in shock, partly in a surge of blinding want.
The cool water was nothing against the fever burning him up. Katsuki manhandled him, turning him, pressing his chest against the rough, artificial rock of the platform. Izuku’s fingers scrabbled for purchase.
“K-Kacchan…”
“Shut up.” Katsuki’s body pressed against his back, the hard line of his cock—smooth, tapered, alien—sliding between Izuku’s thighs. He rocked forward, a slow, grinding promise. One webbed hand splayed across Izuku’s stomach, pulling him back, while the other gripped his hip hard enough to bruise.
There was no preparation, no gentleness. Katsuki spat into his own hand, a crude, wet sound, and slicked himself. He positioned the tip, a hot, insistent pressure against Izuku’s entrance.
Izuku tensed, a flash of fear spiking through the lust. “Wait—”
He didn’t. Katsuki drove forward in one brutal, shearing thrust.
Izuku screamed. The sound was torn from him, raw and echoing in the vast chamber. It was a blinding, white-hot rupture of pain, so intense his vision swam. He felt stretched, split open, filled beyond capacity.
Katsuki froze, buried to the hilt, a shudder racking his own frame. A low, continuous growl vibrated through his chest into Izuku’s back. “Fuck… fuck… tight…”
The pain began to recede, metabolizing into a deep, overwhelming fullness. Katsuki gave a tentative, experimental roll of his hips. The friction sparked something else, a devastating pleasure that burned right at the core of the ache. Izuku sobbed, pushing back, a silent plea for more.
That was all Katsuki needed. He set a ruthless, pounding rhythm, each thrust jarring Izuku forward against the rock. The water churned around them. Katsuki’s claws dug in, his teeth finding the juncture of Izuku’s neck and shoulder, biting down—not enough to break skin, but to claim. Every grunt, every snap of his hips, was a punctuation mark of ownership.
Izuku was lost. The world narrowed to the slam of their bodies, the smell of salt and sex, the animalistic sounds Katsuki was making in his ear. His own cock was trapped, hard and leaking against the rock, the ignored friction a sweet torment. He was babbling, a stream of broken words: “Yes—god—Kacchan—please—”
Katsuki’s pace became erratic, frantic. His tail thrashed, smacking the water’s surface. He sealed his mouth over the bite mark on Izuku’s neck and roared as he came, a hot, pulsing flood deep inside. Izuku felt it, the intimate, impossible heat, and it tipped him over the edge. His orgasm ripped through him, silent and devastating, leaving him trembling and blind.
For a long moment, there was only the sound of their ragged breathing and the gentle lap of water. Katsuki remained lodged inside him, his body a heavy, possessive weight. He nuzzled the bite mark, his breath hot.
Izuku’s legs gave out. Katsuki held him up, withdrawing slowly, a sensation that made Izuku gasp. He turned Izuku in his arms, his crimson eyes searching the dazed, freckled face. His usual scowl was gone, replaced by a look of fierce, bewildered satisfaction.
Before either could speak, a movement caught Izuku’s bleary eye.
Deep in the kelp, at the far, dark end of the enclosure, a shadow detached itself. Sophia. Her large eyes were wide, reflecting the tank lights for one heartbreaking second before she turned. With a powerful, silent flick of her thick tail, she vanished into the blackness of the artificial deep-sea chasm, a place not even Katsuki ventured.
She had seen everything.
The warmth of the afterglow drained from Izuku, replaced by a cold wave of guilt. “Sophia…”
Katsuki followed his gaze, his post-coital haze sharpening back into a glare. “Forget her.” But his voice lacked its usual conviction. He looked from the chasm back to Izuku, his grip tightening.
A sudden, different alarm cut through the quiet—a sharp, persistent ping from the main console. The sonar grid. Something large had just breached the perimeter nets, a half-mile out from the cove. Something moving fast, and heading straight for the station.
Izuku’s blood went cold. It wasn’t a ship signature. The shape was all wrong.
It was biological.
The sonar’s ping was a spike through Izuku’s brain. Biological. Large. On an intercept course.
“What?” Katsuki snarled, his head whipping toward the console. The post-coital haze evaporated from his eyes, replaced by instant, razor-sharp alertness. He released Izuku, his tail flicking once to put distance between them, his body coiling into a defensive posture. Every fin stood erect, the lionfish spines a threat display.
Izuku scrambled out of the tank, water sluicing from his body onto the tile floor. He stumbled to the console, his legs shaky, the ache between them a distracting, shameful throb. The screen displayed the sonar feed in ghostly green lines. The shape was unmistakable—not a whale, not a shark. It had the sinuous, multi-limbed outline of something that shouldn’t exist. And it was trailing three smaller, mechanical signatures. Boats.
“Oh, god,” Izuku breathed, his fingers flying over the keyboard, pulling up the coastal surveillance cameras. The feed from the outer bluff was grainy, washed in night-vision green. Two low-slung, black inflatables with muffled engines, and a larger trawler further back. Figures moved on deck, loading something that glinted—a net launcher.
“Who is it?” Katsuki was at the surface, his hands gripping the edge, knuckles white.
“Poachers,” Izuku said, the word tasting like ash. “Hybrid catchers. They’ve been hitting stations down the coast. They’re looking for exotic species. For experiments, or…” He couldn’t finish. The rumors were too vile: private collectors, black-market biotech, twisted men with even more twisted desires for unique pets. “They strive to kidnap mer for their own twisted pleasures.”
Katsuki’s eyes blazed. “They’re coming here?”
The console lit up with another alert—a perimeter sensor on the seaward side of the facility had been tripped. They were already in the cove.
“Sophia!” Izuku gasped, lurching back toward the tank. She was in the chasm, blind to the threat. “Kacchan, you have to warn her, you have to get deep!”
“I’m not hiding in a fucking hole!” Katsuki roared. But a flicker of fear crossed his face—not for himself, Izuku realized, but for her. The dark, quiet one he pretended to disdain.
A thunderous BOOM shook the entire station. The lights flickered. From the direction of the lower access hatch—the reinforced door that led from the cove into the filtration systems—came the sound of wrenching metal.
They were breaching.
Panic, cold and final, seized Izuku’s throat. He was a scientist, not a soldier. His eyes darted to Katsuki, to the dark water where Sophia had vanished, to the shuddering door.
“The cave,” Izuku hissed, running to a wall panel and slamming his hand on a red switch. “The back cave, it’s shielded, it has a lock from the inside!” It was a last-resort shelter, a holdover from the station’s more paranoid founders.
Katsuki bared his teeth. “I said I’m not—”
“It’s not for you!” Izuku screamed, raw and desperate. “It’s for her! Get her and go! Now!”
Another crash. The lower hatch groaned. Voices, human, shouting commands.
That did it. Katsuki met Izuku’s eyes for one searing second—a storm of rage, possession, and something terrifyingly like regret—then dove. He cut through the water like a missile, a streak of orange and red, vanishing into the kelp forest toward the deep chasm.
Izuku stood alone, dripping, half-naked, in the humming room. He had maybe a minute. He grabbed a heavy wrench from a toolbench, his hand slipping on the cool metal. It was pitiful.
The lower hatch blew inward with a final, deafening shriek.
Three men surged through, dressed in black wetsuits, faces obscured by tactical masks. They moved with brutal efficiency. One covered the door. Two swept the room with rifle-mounted lights. The beams sliced through the dimness, catching the swirling silt in the tank, the maps on the wall, and finally, Izuku, frozen by the console.
“Well, look what we’ve got,” the lead man said, his voice distorted by a rebreather. The light pinned Izuku. “The keeper. Where are the fish, kid?”
Izuku’s mouth was desert-dry. He said nothing. The wrench felt like a toy.
The man’s gaze scanned the massive tank. It looked empty. “Check the water. They’re here. The sonar don’t lie.” He stepped toward Izuku. “You can make this easy, or you can make it fun.”
From the corner of his eye, Izuku saw a ripple in the shadowed mouth of the artificial cave. Not the deep chasm—the smaller, back-up shelter. A flash of dark scale. They’re in there. They made it.
He had to sell it.
“Gone,” Izuku choked out, forcing a tremble into his voice. “Transferred them… last week. For study. You’re too late.”
The man was inches away now. He smelled of salt, sweat, and gun oil. He laughed, a low, ugly sound. “Bullshit.” He backhanded Izuku across the face.
The pain was bright and stunning. Izuku crumpled to his knees, the wrench clattering away. Copper blood filled his mouth.
“Search the water,” the leader barked. “Use the probes. Shock the little fuckers out if you have to.”
One of the other men moved to the tank’s control panel, starting to override the systems. The gentle hum of the pumps changed pitch, becoming a menacing whine. They were going to electrify the water.
“No!” Izuku screamed, lurching forward.
A heavy boot connected with his ribs. He gasped, curling in on himself, the world graying at the edges. He saw the man raise a strange, rifle-like device—a net launcher—and aim it at the tank, sweeping it toward the kelp forest.
Then, from the mouth of the back-up cave, a tendril lashed out.
It was fast as a whip, dark and powerful. It wrapped around the ankle of the man with the net launcher and yanked. He shouted in surprise, firing the weapon wild. The weighted net shot upward, tangling uselessly in the lighting rig. The man was dragged, scrabbling, toward the water’s edge.
“What the fuck—”
A second tendril shot from the darkness, then a third. They seized the man’s arms, his torso, and with a terrifying, fluid strength, pulled him over the lip and into the tank with a massive splash.
The water erupted.
It wasn’t Katsuki. It was Sophia.
She surged from the cave, no longer hiding. Her eyes were voids of black fury, her usually soft features set in a mask of primal rage. Her chubby, powerful body was a bolt of dark lightning. She wrapped the struggling poacher in a crushing embrace, her tendrils pinning his arms, her main tail coils squeezing his legs. He screamed, bubbles bursting from his rebreather.
“Shoot it! Shoot the bitch!” the leader yelled, raising his own rifle.
But before he could fire, a blaze of color erupted from the deep chasm.
Katsuki.
He moved with a speed that defied the water. He didn’t go for the man in Sophia’s grip. He went for the one at the controls. Launching himself half-out of the water, he grabbed the man’s arm and wrenched him forward. There was a sickening crack followed by a scream as Katsuki’s venomous spines found purchase in the man’s shoulder. The poacher convulsed, his body seizing as the neurotoxin hit his system. He slumped, paralyzed, onto the console.
The leader swung his rifle toward Katsuki.
Izuku, on the floor, saw it happen in slow motion. He saw the red laser sight dance over Katsuki’s chest. He saw Sophia, still grappling with her prey, turn her head, her eyes wide with horror.
“NO!”
Izuku didn’t think. He launched himself from the floor, not at the man, but at the toolbench. His hand closed around the only thing within reach—a long, stainless steel pole with a hooked end used for adjusting filters.
He swung it like a bat.
It connected with the back of the leader’s knees. The man cried out, his shot going wide, shredding a panel of monitors in a shower of sparks. He stumbled, turning his rage on Izuku.
But a second was all Katsuki needed.
The lionfish hybrid was out of the tank in a terrifying, fluid arc. He used his powerful tail to propel himself across the wet floor, closing the distance in a blink. He slammed into the leader, driving him to the ground. The rifle skittered away.
It was not a fight. It was an execution.
Katsuki pinned the man, his webbed hands closing around his throat. His face was inches from the poacher’s mask, his teeth bared in a silent, feral snarl. The man gagged, clawing at Katsuki’s arms, his kicks growing weaker.
“Kacchan!” Izuku yelled. “Don’t!”
Katsuki’s crimson eyes flicked to Izuku. The murder in them was cold and absolute. This thing had come to steal them, to put them in cages, to do god-knows-what. The man’s face was turning purple.
In the tank, the poacher Sophia held had gone limp. She released him, letting his body float, and stared at the scene on the floor, her tendrils recoiling slowly.
With a final, disgusted snarl, Katsuki changed his grip. Instead of crushing the windpipe, he brought his forehead down in a vicious, stunning blow against the man’s temple. The poacher went still, unconscious.
Silence descended, broken only by the sputtering of shattered electronics, the lap of water, and the ragged, panting breaths of the three of them.
Katsuki rolled off the man, his tail leaving a wet smear on the floor. He looked at Izuku, at the blood on his mouth, the blossoming bruise on his face. A strange, choked sound left his throat.
Sophia slipped silently to the edge of the tank, her large eyes taking in the wreckage: the two unconscious men on the floor, the one floating face-down in her water, the paralyzed one drooling over the console.
Izuku hugged himself, shaking. The adrenaline was bleeding away, leaving him cold and hollow. He looked from Katsuki’s heaving chest to Sophia’s stunned face.
Outside, through the blown hatch, the sound of an engine gunned to life echoed across the cove. The trawler. The third boat. They’d heard the commotion. They were leaving.
For now.
Katsuki dragged himself back to the tank and slid into the water, a slow retreat. He didn’t look at Sophia. He just stared at Izuku, his expression unreadable.
Sophia finally spoke, her voice a soft, shattered thing in the ruined quiet. “They saw me,” she whispered. “They know I’m here.”
Izuku understood. The catchers had failed tonight. But they had succeeded in one thing. They had shattered the fragile, secret world of the Tidepool Station. They knew. And they would be back.