Author's Note: If you haven't seen the movie and intend to do so, do yourself a favor and hit that back button. What comes next is full of extremely spoilery spoilers.
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Seriously, the movie's good. Go watch it.
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Alright! I loved the movie. It was fun and shiny. It also destroyed the space-time continuum, left everyone traumatized (more or less), gave us the badass mystical Michelangelo we deserve, as much as a Casey more mature than everybody else, including the one turtle he's calling Sensei. Such delightful treats inspired me to write a thing, that I'm posting so you can all suff–enjoy the ride with me.
Prologue
Donnie
The world has been saved. The Kraang aren't a threat anymore, thanks to him and his brothers. They've sent the Technodrome and its master to a prison dimension; they've awaken to a mystical power of unbelievable strength.
He was a starship. He was it; he was its power, its presence in the sky, flying like he was born for it. Because he was born for it.
Now he knows what he can do; how he can merge his knowledge, his tech, and the mystical power that burns inside of him. He can't wait to seriously begin to investigate it. As soon as they'll be done celebrating, as soon as Casey's presence and his wonder at their world lose their novelty, he'll dive head on in a set of experiments like the world has never seen.
Mystical tech. The possibilities are infinite, and Donnie intends to find them all.
He'll build rescue drones that can be mind-controlled. He'll create portals directed at the subcutaneous trackers he has implanted in his brothers' bodies. He'll invent healing medicines that work in a second.
They've almost lost Raph. They've almost lost Leo.
He doesn't want to think about it; refuses to remember the tears in his eyes, the emotions in his heart.
He'll make sure it never happens again.
Raph
They're alive.
Raph has no regrets; not for his chipped shell, not for the scars around his eyes.
They're alive, his brothers, his beloved, adored brothers, the ones he's taken care of since as long as he can remember, the ones he worries about each time they're out of his sight–and sometimes even when they aren't.
And Leo understands. Leo apologized, from the depths of his heart, and Raph knows he's sincere. Now Raph won't have to try to go through him all the time; he won't meet a wall instead of the brother he loves, he can relax and enjoy his newly saved life.
His brothers are all around him, in the lair, safe and sound.
He's the proudest big brother in the world.
Leo
He pretends he's sleeping. Maybe that way, sleep will actually come visit him; maybe that way, he'll forget his mistakes and the smugness that almost cost them the world.
Now he understands what it truly means to be a hero. He understands the selflessness that Raph has always tried to teach him.
Only two days ago, he would have felt proud for what he accomplished, for the way he rose to the challenge and contributed to the Kraang's defeat.
Not anymore.
His family is alive; that's enough. He can laugh with them and tease them, he can take part in the ridiculous challenges they all love so much. He can let the joy of their presence eclipse everything else. He'll let them know how much he cares about them; he'll better himself for them.
He won't tell them about the coldness and the darkness of the prison dimension, about the way he thought he had lost them forever–and about the peace he felt, thinking he had given his life for them. He won't mention the feeling that his childhood is gone, or the new fear in his heart–the fear that another mistake could kill them.
For the first time in his life, he doesn't intend to complain.
It's not about him.
Mikey
In the dim light of his desk lamp, Mikey looks at his hands. His wonderful, amazing hands. He's always known that they could reach into the void to create, to turn the visions of his mind into reality. From them new worlds could be born; have been born.
His hands have saved Leo. They bring life; they bring joy.
Mikey wants to unleash their full power. He doesn't know how to do it, yet; but he knows he'll make it.
It's only a matter of time.
Splinter
He tosses and turns in his bed, unable to sleep.
He remembers the atrocious minutes when he's thought his son lost forever. The pain that tore his heart apart; the void in his thoughts. April's presence next to him, the warmth of her hand on his back.
He remembers the minutes before that, hearing about his son's choice; his baby Blue sacrificing himself so everyone else could live.
He thinks about his own choice to turn his back to his grandfather because he blamed him for his mother's sense of duty. He chased fame and money, until he became a rat; after what he raised his four sons with love and TV shows, and zero idea of what he was doing.
He taught them to have fun and to enjoy themselves, and now they're learning sacrifice and selflessness–and he's terrified.
They're becoming more like their grandmother than he has ever been, and he knows what it entails.
He can't lose them.
April
She sleeps peacefully. All is well in her world; a world that they've saved, again. Her family is safe, united once more, with the new addition of Casey Jones.
Casey Jones who's apparently Cassandra's son. Or would have been. She's still confused about the whole coming-from-the-future thing; she's asked Donnie, who muttered something about the downfall of the space time continuum before ditching her to beat Raph's new record instead of trying to explain, a sure sign that he's as confused as her.
Still, everything will be okay now. She's sure of it. She'll make sure of it, with all her will and all her might.
Commander O'Neil. It does have a nice ring to it.
Casey
The first night, he slept well. He was exhausted for sure; saving the world can do that to a body. If he dreamed, he doesn't remember it.
The second night, however…
He wakes up drenched in sweat. The vivid images stay with him; the golden light that swallowed the shards of Master Michelangelo, the red light that erased Master Leonardo from the surface of the tortured Earth are burnt into his retina.
He's seen so many die.
He's managing not to think about it during the day. His new friends have undertaken to show him their world; his world, before the Kraang came and destroyed everything. Almost everything.
Here, everything is new to him: the people, the food, the sky devoid of any Kraang threat. He loves it, every bit of it.
And still, in the darkness of the turtles' lair, in that unfamiliar room whose bed is comfier than anything he has ever laid on, his stomach filled with the most delicious food he's ever tasted, safer than he has ever been, the truth hits him. Hard.
He's not from this world. He's the only survivor of his kind; the only witness of humanity's downfall.
Humanity will survive, thanks to him and the turtles and their family. He has seen how great they are, how powerful. He's found Leonardo again, and he calls him Sensei–it feels natural.
But this Leonardo isn't his Sensei, not really.
Casey has lost everyone. His mother, his friends, his Sensei. His world. It was a world of fire and tears, of burnt earth and fickle hope, but in it he found love and guidance.
At night, Casey Jones crumbles. His tears soak the soft sheet; his shivers shake the bed.
He's never been less alone, and he's never felt so lonely.