May contain spoilers up to episode 13 of the anime.
This was inspired by a cute little pic on tumblr/deviantART but which it seems won't let me link to. The artist's dA screenname is Riocakes and the picture is titled "Once Upon a Time", if you want to try and find it. This fanfic is on dA too but I won't bother directing you to it since you're already reading it here.

Shingeki no Kyojin and its lovely characters belong to Hajime Isayama, not me~
~ ~

Once upon a time, there was a little boy named Armin…
Armin loved to read. He would read all day about knights and dragons, invincible heroes and far-off lands. He read every book he could get his hands on. This little boy read so much, that one day he ran out of books. When even revisiting his favorite tales gave him no new pleasure, gloom settled over him like a thick, lonely cloud.
When his grandfather realized this distress, he dug out from the depths of his closet a musty, leather-bound book, yellow pages crumbling with age. He declared it a very, very special book, so special that even his possession of it was a secret. With this, he presented it to his grandson. Armin accepted it with wide eyes, handling it delicately like a newborn baby. He dove into the pages intently, not knowing what to expect. What he discovered within exceeded his wildest imagination.
This book told of endless stretches of salty water, unfathomably deep and wide. It spoke of vast expanses – plains of sand – whose skies blazed with the sun, but at night shimmered with millions of stars. The faded letters painted swaths of ash and liquid fire, whispered tales of frozen ranges composed with ice. There were grand mountains that rolled into hills, which then glided down to sweeping meadows.
Instantly, Armin was captivated, so much so he could barely put it down to see if there was more. More books that told of the world outside the walls, of these fantastic lands nobody alive today had seen. But when he asked his grandfather, he said he had only the one. Armin was crestfallen. He dearly desired to see these places for himself, and he would have if there were not one gaping problem.
The walls.
Long ago, humanity used to live in peace, free to live wherever they desired. But then, a hundred years ago, terrible monsters appeared out of nowhere. They towered over everything and devoured every human they laid eyes on. The earth shook as they lumbered blindly toward shrieking crowds, driven by an insatiable, greedy hunger for human flesh. They were immortal; cannons were worthless, nothing could stop their advance. Thousands died in vain, many more perished as they tried to flee. Nowhere was safe. These monstrosities were called Titans, and none escaped their terror.
Humanity's only choice was to run. They fled behind three great walls, forced to live like cattle. Only the most courageous dared leave this "haven", and often at the cost of their lives. Many heralded them as heroes, but the rest named them martyrs, useless tragedies in an idealist dream.
Armin Arlert was born and raised in this humiliation. He knew nothing but the fragile shelter of the walls. To those around him, life outside them was nothing but ancient history, irretrievable and better left forgotten. But it made him restless and nervous, existing as a caged rabbit quaking before the hunter.
Armin saw the world differently than others did. He knew life beyond the walls was possible, the problem was, it would take incredible courage, sacrifice, and an extravagant helping of pure luck. It was a risk that most people weren't willing to take. Frankly, Armin could understand – even the crude sketches of Titans in history books sent a chill up his spine. But when he saw that book, something changed. He didn't want to be scared anymore. He hated what was, he craved what could be. To make a stand, to see the outside world, to be a lionheart…it was a wild energy, like a roaring river, threatening to sweep him away.
Immediately, je flew to his best friend, Eren. He could barely contain his enthrallment, babbling almost incoherently about this wonderful, breathtaking world, about all these vast expanses, open and so wide you could run forever and never reach an end. And oh, how he wanted nothing more than to see it! They could even go together! If only…
Armin's ramble slowly trickled into silence. He stared at his friend, praying that Eren, so passionate, would understand. But he said nothing. Armin's heart sank. What had he done this time? Was he being too eager? Too optimistic? Or had he just gone too far…?
Then Eren smiled. Armin hesitated, not daring to believe it. But the next thing he knew Eren was hammering him with questions, most of which he couldn't even answer. And that was all it took to set it off.
Every day, for the next several years, they would fantasize about what they were going to do when all the Titans were gone, dreaming of the day when they would no longer be locked within their cage. The two would revel in the thought, eager to join the heroic Scouting Legion, the soldiers who dared to make a difference. Even Mikasa, Eren's adopted sister, displayed interest, though Armin secretly mused that it probably had more to do with sticking close to Eren.
It was in every sense a normal day, in their home district of Shiganshina. The sun rose, just like always, filling the town with warmth. The streets bustled. Birds sang. Children played and squealed. Mikasa, Eren and Armin were by the river, once again considering their lives. They knew this false security couldn't last forever.
It's a funny thing, fate is.
This was the year 845. It was the day when humanity met its twilight.
A resounding crack split the air, and the earth shuddered. Slowly, a gargantuan, humanoid figure loomed over the top of the wall. Its skinless face was shrouded in steam, deep-set eyes peering at the motionless humans trapped inside. The silence was like a guillotine; it hovered above the heads of the people, then fell with the wall in a blast of rock.
Thousands fell prey to the Titans that day. Eren's mother, Carla Yeager, was one of them. The boy swore bloody vengeance, but all three of them knew that nothing would ever be the same again. No amount of delusion or begging could ever bring those precious lives back. It was too much to imagine, that in less than a day they could be pushed so close to the cliff, its greedy, fanged drop yawning right before them. That day, the sun set in a crimson tide.
The moment he turned twelve, Eren swore his allegiance to the military, prepared to sacrifice his life to the war which could see no end. Armin and Mikasa too offered their hearts. They possessed no promise of survival; a feeble dream was their only link to hope.
Three years later, the predator advanced again. Just like before, the gate shattered in a hail of rubble, and the Titans hungrily swarmed the city. Mankind's foolish attempt at defending their fragile lives failed. The ground was saturated with blood. Humanity wailed for a savior, but none appeared.
Then suddenly, in the form of a Titan that destroyed its own kind, a tiny sliver of light broke through the carnage. This light was a monster who took the form of a young man – Eren Yeager. The people knew only one thing for sure: this was either bane or boon. But if this new power was one they could control…
They had a chance. A real, fighting chance.
And they seized it.

To this day, lives are still taken. Blood is still shed. A chance is not a promise. But Eren, Mikasa, and Armin aren't the helpless little kids they were five years ago. They've grown up. They're learning, growing stronger. They have courage, they have drive, and they have talent. The odds, so odiously fickle in a world where death is rampant, are in their favor.
And someday, they will live to see the outside world.

~ ~Do you have any idea just how hard it is to write in full intensity with some else's story?! D: I kept thinking of ways to make it more intense, and just as I was about to let the energy flow...
"oh"
"oh yeah"
"this happened, not that"
.-.
OTL

But I figured, it's been three weeks or so, I've been editing it obsessively for the past few days and honestly I think it's as good as it's gonna get at this point without a total rewrite. So here's hoping my effort paid off and this is intense enough for you guys. Anyways, thanks for reading! ^^