Chapter Text
Senku had always known his body wasn’t the strongest. He’d known it was failing him, that it was only a matter of time before it shut down completely.
His father had always tried to comfort him, always insisting that he’d be okay, no matter what. Senku knew that wasn’t true. He could see it in the way the old man’s hands would linger just a second too long on his shoulder, in the way his voice would tighten whenever doctors spoke for too long. He saw the tears too—always there, always threatening to spill over, but never allowed to fall. Not where Senku could see, anyway.
Hospitals became a constant. He was in and out so often that a full week of school felt like an achievement rather than a routine. Most of the time, his body simply couldn’t handle it.
There were tests. Countless ones. Blood work, neurological exams, machines humming softly as doctors searched for answers that refused to appear. Nothing ever showed up. Not until the scans.
He was still a child when they finally gave it a name. All the symptoms, all the unexplained failures of his body, led back to one simple truth—his brain was failing him.
Senku didn’t cry easily. Never had.
But he remembered that day clearly. Remembered crying until his chest hurt, fingers knotted tightly in Byakuya’s jacket as if letting go would make it real. He remembered thinking—irrationally, desperately—that it had to be a mistake. That the most vital organ he had, the one thing he relied on more than anything else, couldn’t possibly be the thing betraying him.
Neither of them were religious. Science and facts had always been enough. And yet that night—alone, sitting at his bedside—Byakuya prayed. He didn’t know who he was praying to, or what he was asking for. He just knew he didn’t want it to be true.
Senku’s childhood wasn’t terrible. Even after the diagnosis, he still went to school when he could. He still learned, still absorbed knowledge like his life depended on it—because, in a way, it did. He crammed facts and theories into his head relentlessly, determined not to waste what time he had left.
Things didn’t truly worsen until his teens.
Hospital visits became daily. Sometimes they lasted hours, sometimes weeks. School turned into a screen, then into something he barely interacted with at all. Eventually, he dropped out and continued online—when he was well enough. Some days he felt like he could build seven rockets without breaking a sweat. Other days, standing was impossible.
It was infuriating.
There were experiments he wanted to run, theories he wanted to test—things he could see so clearly in his mind but could no longer bring into reality. So he adapted. If he couldn’t do them himself, he had others do them for him.
Byakuya was often busy with astronaut training by then. He hadn’t planned on applying originally—not until Senku’s relentless arguments and carefully constructed speeches wore him down. One of us has to go, Senku had said. Eventually, the old man cracked.
So it was Taiju and Yuzuriha who followed Senku’s instructions instead. Taiju carried them out with unwavering dedication, even when he didn’t fully understand what he was doing. And Yuzuriha documented everything—careful handwriting, precise measurements, no detail overlooked. She understood how much it mattered. Maybe more than Senku wanted to admit.
Every piece of knowledge Senku learned, he wrote down. No matter the hour. He filled notebook after notebook, sacrificing sleep without hesitation. When his hands began to shake, or when a nurse stepped in to force rest upon him, only then did he stop.
Senku learned quickly that the most unforgiving variable was time.
Doctors spoke carefully, words dulled to avoid cutting too deep. Unpredictable. Managing symptoms. Focusing on quality of life.
It wasn't hard for Senku to translate.
There was no cure.
It would only get worse.
The clock was already ticking—and it wouldn’t wait for him.
So he planned.
If his illness was going to dismantle his body and mind, he would work around it. He mapped out his days down to the second. He tracked when the dizziness peaked, when the headaches eased just enough for clear thought. He learned exactly how far he could push himself before exhaustion tipped into something dangerous.
Sleep was inefficient.
Eating was negotiable.
Pain was tolerable.
Knowledge, however, was non-negotiable.
Hospital rooms became laboratories in his mind. Machines hummed softly as IV lines tugged at his arm, and Senku dismantled them mentally—breaking each one down, reconstructing it piece by piece. He memorised textbooks. Re-read notes obsessively. He didn’t know when his brain would finally fold in on itself, so he made sure it would be full when it happened.
Nurses joked about the stacks of books piled in his room, teased him about outgrowing the hospital library. Senku would grin and tell them he was just optimising storage.
What he never told anyone was that he re-read his notes constantly out of fear. Terrified that one day something familiar would suddenly feel foreign.
He had practically moved into the hospital. Byakuya was away most days, and it wasn’t safe for Senku to be alone for long. His episodes came without warning—one moment sharp and steady, the next his vision blurring, thoughts splintering like glass under pressure before everything went dark.
The world outside shrank. Friends became voices, then messages, then nothing at all. Everyone drifted away—except Taiju and Yuzuriha. They stayed. Every visit, every chance they got. After school, on weekends, sometimes until visiting hours ended. Occasionally, staff took pity on them and let them stay longer. A few times, they were even allowed to spend the night—as long as they were quiet.
And sometimes, they barely made it through the door before Senku had an episode and they were forced to leave.
They never complained.
Every time Senku needed something, they were there. He hated how much he depended on them for things he should have been doing himself.
He was grateful anyway.
His father worried constantly. No matter how carefully he tried to hide it, Senku always noticed. The way his voice tightened over the phone. The way he looked just a little more exhausted every time they met. They didn’t see each other often—Byakuya spent most of his time in the U.S. training—but whenever he had leave, he spent it with Senku.
Every visit came with the same conversation. Byakuya insisting he could drop out. Senku telling him to shut up.
If either of them was going to reach the stars, it would be Byakuya. And Senku would make sure he lived long enough to see it.
Even if he had to watch from a hospital bed.
Time was cruel.
But Senku Ishigami would never lose a battle just because the odds were bad
26 May 2019
This would be the last time Byakuya would see his son before leaving Earth.
He was excited—of course he was. It had been his dream long before it had ever been Senku’s, and somewhere along the way, the two had fused into one shared ambition. Space. The stars. Proof that humanity could reach beyond itself. Now he was finally doing it, not just for himself, but for the both of them.
He promised souvenirs. Said I love you more times than he could count. His eyes were already red, already sore from crying, and he hated that Senku noticed anyway.
He didn’t want to leave. Every instinct in him screamed against it. But this was what he had trained for—two long years of preparation, sacrifice, and relentless testing. And Senku had been adamant. Almost frighteningly so.
You’re going, he’d said. Like it was a fact. Like anything else was unacceptable.
Byakuya made sure everything was in order before he left. Nurses briefed. Emergency contacts confirmed. Instructions written down, rewritten, checked twice. He went over it all again in his head, searching desperately for anything he might have missed.
Nothing.
And yet.
The feeling wouldn’t go away.
It sat heavy in his chest, lodged in his throat, a quiet pressure that made it hard to breathe. A sense of wrongness he couldn’t explain away with logic or reason. He tried to tell himself it was just nerves. Anxiety. Fear of leaving.
But fear didn’t usually feel like this.
Had he forgotten something? No—surely not. And even if he had, it couldn’t be anything important.
So why did it feel like he was walking away from something he would never get back?
27 May 2019
The plane back to the U.S. was preparing to land.
It had been a full day, and the feeling was still there.
In hindsight, that should have been the first sign.
He sat rigid in his seat as the ground rushed up to meet them, hands clenched tight in his lap. A part of him—small, desperate, irrational—wondered if he should have stayed. If he should have turned around, ignored everything else, and gone back to Senku’s bedside.
But he hadn’t.
Because staying would have meant disappointing his son.
And Byakuya didn’t think he could survive that.
The moment the plane touched down, the pressure in his chest worsened. He barely had time to process it before a worker from NASA was waiting just beyond the gate, waving him over with practised efficiency.
Introductions were brief. Polite. Professional.
He was guided toward a black SUV before he could even pause to collect himself.
Everything moved quickly after that. Too quickly.
Security checks blurred together. Paperwork. Briefings. Signatures scrawled with a hand that didn’t quite feel steady. He answered questions on autopilot, mind drifting constantly back to the same image—Senku, alone in a hospital room, surrounded by machines that tracked the slow betrayal of his body.
He checked his phone whenever he could. Again and again. No messages. No missed calls.
That night, once he was finally alone, he called.
It was late. 9:46 p.m. for him—nearly 12pm for Senku. He hesitated, thumb hovering over the screen, before pressing call anyway.
It rang three times before he answered.
“—you landed?”
Senku’s voice was thin, but unmistakably bright, stretched too tight like he was forcing himself awake.
“Yeah,” Byakuya said, relief crashing through him. “Safe and sound. You holding up okay? No bad episodes, right? I mean, I’m sure NASA would understand if I—”
“Old man.”
The interruption was sharp, familiar.
“You are going on that rocket,” Senku said firmly. “I’m fine. Stop worrying and focus on not screwing this up, alright?”
Byakuya laughed. He couldn’t help it.
But the knot in his chest didn’t loosen.
28 May 2019
The day passed in a strange, suspended state.
Final medical checks. Final training refreshers. Instructions drilled into him until they echoed in his skull. Everything felt heavy with finality. With the undeniable truth that there was no turning back now.
This was it.
That night, he called Senku again.
They didn’t talk long. Senku sounded exhausted—no matter how hard he tried to mask it, the strain slipped through. It didn’t stop him from asking questions. Dozens of them. Rapid-fire, precise, hungry for answers.
Byakuya could hear nurses moving in the background. Equipment shifting. Quiet voices that cut in and out between Senku’s words.
Still, he answered every question. Every last one.
“Get some rest,” Byakuya said gently when Senku’s voice finally began to fade. “I’ll call you after launch, okay?”
“Yeah,” Senku murmured. “You better not back out.”
29 May 2019
Launch day.
It arrived too quickly, like the world had skipped ahead without asking if he was ready. Excitement warred violently with fear, with nerves, with that same unshakable sense of dread curling in his gut.
The launch site was loud—alive with movement, voices, machinery—but Byakuya felt strangely detached from it all. As they suited him up, as they guided him toward the spacecraft, as he spoke to the camera, his thoughts refused to leave one place.
One person.
Just a little longer, he told himself. I’ll be back before you know it.
The countdown began.
Strapped into his seat, surrounded by metal and screens and noise, Byakuya closed his eyes for just a moment.
“You better be watching,” he whispered.
The engines ignited.
And somewhere miles away sat Senku,watching from his phone, Taiju and Yuzuriha on either side.
2nd June, 2019
It felt as though time had stopped behaving properly.
Senku noticed it in the smallest ways at first. Seconds stretched too long, then vanished entirely. He would blink and suddenly a nurse he hadn’t even realised had entered the room would be standing beside his bed, clipboard in hand, asking him questions he was certain he had already answered.
Other times he would be halfway through a thought—an equation, a theory, a memory—and it would simply… drop. Like a wire being cut.
He wasn’t used to this.
It had never happened before.
Pain was familiar. He could handle pain. Fatigue, dizziness, even the constant pressure that made it feel like his skull was being crushed inward—those were variables he had already accounted for.
But losing pieces of himself?
That was new.
That was dangerous.
And he did not like it.
His hands shook constantly now, too badly to form anything readable.
He tried anyway.
The pen slipped from his fingers, he couldn't seem to grasp it.
Again.
And again.
Over and over—until it clattered uselessly against the tiled floor.
Senku didn’t move to retrieve it. He just stared at where it had fallen, chest rising unevenly as he struggled to draw in a proper breath. His vision swam, the room tilting just enough to feel unreal. On the nearby shelf lay an open notebook, pages filled with neat, compact handwriting from only days ago.
His handwriting.
Precise. Familiar.
“…Senku? You… okay?”
Yuzuriha’s voice pulled him back. She stood near the foot of his bed, worry etched into every line of her face, one hand hovering uncertainly while the other twisted the hem of her sleeve. Taiju hovered beside her, doing his best—and failing—to look calm.
“Yeah,” Senku replied automatically. “I’m fine.”
The words came out weaker than he intended.
They noticed. Of course they did. They always did.
“Dude,” Taiju said, hesitant, “you do not look ‘fine.’”
Senku huffed softly, rolling his eyes. “Technically speaking, I haven’t looked ‘fine’ in years.”
Yuzuriha smiled at that, but it didn’t reach her eyes. She crossed the room quickly, picked up the pen, and dragged a chair over to sit beside him.
“…want me to write for you?” she asked quietly.
He hesitated.
That alone was enough to answer.
“…yeah,” he admitted after a moment. “Just—don’t mess it up.”
Yuzuriha let out a shaky laugh, blinking rapidly as she nodded. “I won’t.”
So Senku dictated while Yuzuriha wrote, Taiju occasionally interrupting to ask questions. It was slower than Senku liked. Words tangled in his mouth. Thoughts slipped just out of reach before snapping back into place with a dull ache behind his eyes.
At some point Taiju sat on the edge of the bed, fists clenched so tightly his knuckles turned white, listening like every syllable mattered.
Nurses came and went. Doctors spoke in hushed tones just outside the door—close enough for familiar phrases to drift in.
Rapid progression.
Neurological deterioration.
What else can we do?
We’re doing everything we can.
Senku watched the panic deepen on his friends’ faces with every new word. They wiped at their eyes when they thought he wasn’t looking. They didn’t understand every term—but they understood the meaning.
By evening, his vision refused to stay steady. The ceiling lights blurred into soft halos, colours melting together no matter how hard he tried to focus. His head throbbed with every heartbeat, pressure building until it felt like something inside him was going to burst.
Neither of them had left.
He leaned back against the pillow, breath shallow, chest tight.
Taiju noticed immediately.
“Hey—hey, hey,” he said, panic breaking through. “Nurse! Nurse—!”
“I’m not dying yet, big oaf,” Senku muttered. His voice was barely audible. He wasn’t even sure if it was true anymore.
A nurse arrived quickly, calm but alert. She checked monitors, adjusted lines, watched numbers that Senku could no longer make sense of. Her expression stayed composed—but he caught the brief tightening of her jaw.
She murmured reassurances to Taiju and Yuzuriha before gently ushering them out.
“They’ll visit first thing tomorrow,” she told Senku softly. “We’ll let them in early. Just try to rest, okay?”
Rest.
Right.
The room felt emptier without them. Too quiet. The machines hummed and beeped softly at his bedside, tracking the slow betrayal of his body.
He stared at the ceiling.
Thinking was harder now. Thoughts slipped away if he didn’t grab onto them fast enough—and that scared him more than he wanted to admit.
Sleep crept in through the gaps, tugging at his consciousness like a tide he could no longer fight. As he drifted off, the room grew quieter, until only the hum of machines remained—and a fragile, fleeting hope that tomorrow might hurt a little less.
3rd June, 2019
The morning light was harsh against Senku’s eyelids.
Every breath felt heavy. Shallow. Each movement was like lifting a weight he no longer had the strength to carry. His vision flickered in and out of focus, the world bending at the edges.
A soft voice broke through the haze as the curtain was drawn back.
“Senku? I’m sorry for waking you, dear… your friends are here.”
He tried to nod. Couldn’t. He settled for a weak smile instead.
The nurse lingered as Taiju and Yuzuriha were brought in, then stepped just outside the door.
“How… how are you feeling?" Taiju asked. His voice trembled like he already knew the answer.
“Honestly?” Senku whispered. “A lot worse.”
That was all it took.
They could see it—in his colourless skin, the tremor in his hands, the way his chest fluttered unevenly.
“…could you pass me my phone?” he asked quietly.
Taiju handed it over with shaking hands.
Senku typed slowly. Each letter took effort. He didn’t know if his old man was awake. Didn’t have the energy to think about time zones or where he might be floating above the Earth.
He just needed to send it.
One last message.
Something he never dared to say out loud.
Old man
It’s okay —
Keep going forward —
I love you dad —
He stared at the screen for a long moment.
He wished his dad was with him.
But he was glad—so glad—that he had gone to space. He had fulfilled both of their dreams and that's all he could ask for.
Senku set the phone down on his lap. He didn’t close it. He didn’t need to.
A small smile tugged at his lips.
That was enough, it wasn't much but his dad would understand.
Taiju and Yuzuriha clutched his hands, sobbing openly now. They knew what was going to happen.
“We’ll keep everything,” Yuzuriha whispered through tears. “Every notebook. Every experiment. Every little thing—you won’t be forgotten, Senku. I swear.”
“I—I’ll do my absolute best in all my classes!” Taiju choked out. “I promise! I’ll make you proud—I’ll keep pushing, just like you taught me!”
Senku wanted to reassure them. Wanted to tell them it was okay.
But he didn’t have the strength. He could only hold the smile he had plastered on.
His vision blurred completely. Whether from tears or his body’s final failure, he couldn’t tell.
He felt their warmth. Their love. Their hands gripping his like they could anchor him to the world.
Sounds faded. Voices softened. The nurse murmured gentle words he couldn’t make out.
He was scared—of what came after, of what would happen to his body, of whether he had learned enough.
But another part of him was… okay.
He had learned all he could.
And now he was finally going to find the answer to the last question anyone ever asks.
A few minutes later, miles upon miles away, Byakuya Ishigami’s phone buzzed. A smile found its way onto his face as he grabbed it, expecting another routine check-in from his son, instead tears fell onto his screen as he read the words over and over.
“It’s okay.
Keep going forward.
I love you dad.”
The words were few, impossibly simple—but they landed like a meteor, crushing down onto his chest. He felt his throat tighten, his knees go weak.
That gnawing, uneasy feeling he'd carried around with him ever since he left Senku—he finally understood.
It hadn't been worry, not had it been fatigue...it had been a warning.
"Oh," he whispered, voice breaking, barely audible over the ringing in his ears. "So....that was it."
He just got the words out before the first sob came out—loud, raw, unrestrained. Then another. Then another. He couldn't stop.
He still brought back souvenirs.
....
Senku would have loved them.