Chapter Text
It was late in the evening on a Friday when Nagisa had finally made it back to his apartment after a long day of teaching. He let out a sigh as he unlocked his door and stepped inside, taking off his shoes in the entry way and making his way to the bedroom.
Normally, he would prepare dinner for himself, or even begin lesson planning for the weekend ahead while marking his students work, however he was simply too tired to do either of those.
A quick nap wouldn't hurt, and he still had the weekend available to prepare for the school week ahead.
Being a teacher was tiring. Rewarding, but tiring.
“Lucky Koro-sensei, only having to do this for a year…” Nagisa murmured to himself, wearing a soft smile as he wandered to the bedroom.
“Then again, what Koro-sensei did in a year is more than what most teachers do in a lifetime…” He added a moment later.
He was only going to take a quick nap, he told himself, he still had things he wanted to get done.
“The job of a teacher is never done, huh? Man, if only Koro-sensei could see me now,” Nagisa said, letting out a light laugh.
It had been nine years since Nagisa graduated.
Nine years since Koro-sensei died.
In that time Nagisa had become a teacher. It was intimidating at first, but he quickly found his footing. He had taken the lessons he learned in E Class, the lessons he learned from Koro-sensei, and had become a successful teacher.
He hoped Koro-sensei would be proud, he thought to himself as he threw himself onto his bed.
He just wished that Koro-sensei was there to see it.
Not just how far he had come since graduation, but how far everyone had come since graduation.
It was times like this, when Nagisa was tired and beaten down from a long week, that he found himself looking to the past with fond melancholy.
In no time at all, Nagisa felt his consciousness drifting, as he fell fast asleep.
Nagisa was woken up by the sound of an alarm. It wasn’t his current alarm, but one he hadn't heard in years.
He slowly rose up from his bed, tiredly rubbing his eyes as he tried to recall where he had heard that alarm clock sound before.
However, as he lifted his body upright, he felt a chill run down his spine. An unmistakable sensation brushed against the back of his neck.
Long hair.
What tiredness remained shattered instantly as his eyes snapped open.
It had been years since he had long hair. He never let it grow out, and he most certainly never let it get this long.
The last time he had hair this long was back when he was still in high school.
Something was unmistakably wrong.
As he looked around, he realised he was no longer in his apartment.
He was in his bedroom.
His old bedroom.
The one in his mother’s apartment.
“What the…?” Nagisa murmured to himself, convinced he must’ve still been half asleep.
This couldn’t be possible. It just didn't make any sense.
He sat there, in idle confusion, his brain desperately trying to connect the threads.
It wasn't just that it was his old bedroom.
Something else still felt… off.
It looked exactly the way it had when he was still in high school.
As he looked around the room, his eyes fell on a nearby mirror where he caught a glimpse of his reflection.
“That's… there's no way…” Nagisa murmured to himself, as he looked at the boy in the reflection staring back at him.
The reflection of a fifteen year old high school boy, with long blue hair.
The reflection of a Nagisa Shiota, nine years younger.
A few minutes had passed, and Nagisa was fairly certain he had an idea of the situation.
As much of an idea as he could, anyway.
At first he thought this was a dream, or possibly a nightmare, but he was unable to get himself to wake up. Lucid dreaming is quite difficult to pull off and usually you wake up when you realise it's a dream, so Nagisa felt confident he could rule this option out.
However, the alternative hypothesis is that he has, somehow, gone back in time to when he was still in high school.
The idea seemed insane, but all things considered, it wouldn't even make it into the top five of insane things that he has experienced.
He wasn't certain that was the situation, but it was his most viable theory for the time being.
He had also taken a look at today's date. Today was the day that E Class would first meet Koro-sensei.
The day that Koro-sensei would become their teacher, and their target.
Suddenly, Nagisa heard a voice call out to him from somewhere else in the apartment. It was his mum, Hiromi Shiota.
“Nagisa? Are you awake? We can't have you be late to school!” She called out.
For a moment, Nagisa froze up. If his theory was correct, then that means his relationship with his mum has reverted back to what it was when he was still in high school.
Which is to say, not good.
He would have to figure out what to do about that… at some point.
Even though his relationship with his mother had improved somewhat in the nine years since graduation, some scars never truly heal
If she really has reverted back to how she used to be, Nagisa would have to be careful.
For now, he decided to put that issue off until he could confirm some things first.
“Nagisa? You better not still be sleeping! Don't make me come in there!” Hiromi called out yet again.
“O-oh! I'm awake! Sorry, I'll be out in a minute!” Nagisa hurriedly replied back.
It took a bit to recall where he kept his clothing and school belongings, muscle memory doing a lot of the legwork.
It had been nine years, after all.
Eventually, he got himself dressed and ready for school.
It felt surreal looking at his reflection and seeing his younger self, even if he had only been a single centimeter taller as an adult.
“You’re finally up, Nagisa. What took you so long?” Hiromi chided as Nagisa made his way over to the kitchen table.
“Sorry… I guess I was still half asleep?” Nagisa replied as he took a seat.
Even though he expected it, seeing his mother look just like she did nine years ago felt uncanny.
“Well, so long as you're awake when you make it to school. How do you expect to make it back to the main campus if you continue to slack off? I like to think I raised you better than that, so don't disappoint me,” Hiromi responded as she got ready to leave for work.
Nagisa wasted no time finishing breakfast. It wasn't just that he wanted to distance himself from Hiromi as quickly as he could, although that was part of it, but it was because there were a lot of things he wanted to confirm.
“Hah… hah… almost… there…” Nagisa said between heavy panting.
While he could certainly remember how dreadful the mountain climb each morning was, the memory truly did not do it justice.
It had been years since he felt this exhausted. Which, to him, was technically true.
Slowly, Nagisa stumbled his way towards the old campus building, his legs aching all the while.
As Nagisa entered the classroom, he looked around, checking off students in his head one by one.
Rio Nakamura….
Yuma Isogai…
Tomahito Sugino…
It took only a few seconds for Nagisa to finish his surveying. As expected, everyone was accounted for, bar those who were yet to join E Class.
It's possible I'm the only person who's from the future. Nothing seems out of the ordinary, and they all seem to be acting normally…
Of course, it's possible that they're just being cautious, like me.
Nagisa thought to himself as he made his way to his seat.
As he sat down, a girl with green hair and twin pigtails looked over at him with a concerned expression on her face.
“Nagisa, are you… feeling ok? You don't look too good,” she asked sympathetically, referring to how he practically wobbled his way to his desk.
Kaede Kayano.
Or rather… Akari Yukimura.
Back then, she had only been pretending to be their cheerful classmate and friend, trying to get close to Koro-sensei so she could avenge her sister.
Although that would eventually change, right now, Nagisa would have to be careful around her.
If things play out like they did back then, it would be a while before she opens up to everyone.
“I think I must still be a bit tired, the climb up the mountain really did a number on me this morning,” Nagisa replied, draping his arms over his desk in exhaustion.
Suddenly, the door to the classroom slid open, and in walked a yellow octopus-like creature, accompanied by a group of government agents in black suits.
Nagisa felt his eyes go wide as he froze in place. It was Koro-sensei. Even though he expected it, he was still not prepared for it.
It had been nine years since Nagisa last saw Koro-sensei.
Nine years since Nagisa and the rest of E Class killed Koro-sensei.
His vision felt blurry, and it took everything Nagisa had to keep himself composed.
Yet here he was, alive and well, as if nothing happened. Of course it made sense, nothing had happened yet, but it took all Nagisa had to keep his emotions in check as he watched Koro-sensei slither over to the front of the class.
“How do you do? I’m the one who blasted your moon,” Koro-sensei said, introducing himself. “I plan to do the same to the Earth next year. Now I'm your teacher. Nice to meet you.”
Everyone in the class shot a confused look at Koro-sensei. The unanimous opinion back then was that absolutely none of this made sense.
The next to introduce themselves was Karasuma.
“I’m Karasuma, from the Ministry of Defense. First, please understand that what I'm about to say is a state secret,” He sighed. “Let me be blunt: I want you to kill this creature.”
Everything played out exactly as it had back then. Karasuma explained that he needed E Class to assassinate Koro-sensei before the end of the year, or else he would blow up the Earth just as he had the moon.
Of course, most of that isn't actually true.
Nagisa thought to himself with a sigh.
The rest of the day went by without issue. It felt strange being on the receiving end of the classroom after so many years, but it wasn't bad.
In fact, this was a perfect opportunity for Nagisa to relearn the fundamentals. Even though Nagisa was from the future and had already learned all of this content before, it had been a long time since then.
That wasn't to say he had forgotten all he had learnt, but as he paid attention to Koro-sensei's teaching, he couldn't deny that his knowledge on some of this material was rather rusty.
The only benefit was that as he had already learnt this stuff before, so it would be much easier learning it a second time.
Besides, sitting in a lesson without having to worry about planning out the next one, it was actually almost… fun?
I wonder if I'll be able to beat Karma in one of the exams…
Nagisa thought to himself with a smile as he diligently took notes.
Eventually, the day came to an end and Nagisa picked up his things, ready to go home and piece together what he should do from here.
When he got home, he threw himself backwards onto his bed. It had been a long first day back at school, and he had a lot to think about. He didn't know how it was possible, but he really had gone back in time to the very first day everyone met Koro-sensei.
After a moment, Nagisa got himself back up and moved to sit at his desk. He didn't know why or how he had gone back in time, but that didn't matter.
There was something more important that had occupied his mind.
Something that he had already subconsciously decided on when he first arrived here, even if he didn't consciously realise it until now.
Something he had to do.
He reached for a pen and notepad, flipping to an empty page.
As he stared at the blank page, a single thought came to mind.
Save Koro-sensei.
It was an ambitious goal, and Nagisa knew it. There were plenty of ways this could go wrong. If he really had gone back in time, then everything should play out as it had back then.
Or, at least it should, so long as he doesn't do something to seriously alter the past. Even the smallest of change could drastically alter the future.
The butterfly effect, Nagisa recalls vaguely, something Karma once explained to him while they were watching some time travel movie together.
If only I took some notes while we were watching.
Nagisa thought to himself with a light laugh.
He brought the pen down to his notepad, and began writing.
He had two advantages right now: he knew what would happen in the future, and he had several more years of experience than he did the first time.
If he did anything too major, then it could cause this timeline to play out differently, and he would lose that first advantage.
If he wanted to save Koro-sensei, he had to make full use of the tools he had available.
He had to alter the past just enough to save Koro-sensei, that's all he had to do.
But what did he need to change to have that happen?
In his previous run, they had gotten so close to saving Koro-sensei, only having died because he got trapped.
If he could just alter that final scene, nudge it ever so slightly in his favour, maybe Koro-sensei would've lived.
But as Nagisa stared at the page, he realised that there was more to it than that.
He didn't want to save Koro-sensei just for his sake, but for everyone's sake. He wanted Koro-sensei to see how far they all came because of him. He wanted Koro-sensei to see the future that he never got to see.
Not only must Koro-sensei survive, but the Assassination Classroom must survive as well.
Right now, the E Class was nothing more than a collection of failing students with inferiority complexes. It was thanks to Koro-sensei's unique classroom and the lessons he taught them, that they grow up to become successful.
Nagisa couldn't alter that.
That had to remain unchanged.
Before he can convince everyone to help save Koro-sensei, he would have to convince everyone to help kill Koro-sensei.
Just like it had played out in the past.