With a hiss and a click the door behind you shuts.
You've really done it. It's not like you had a lot of choices, but still. You're 18 years old and up until this very moment you spend each and every moment of those 18 years inside the Vault, securely hidden from outside world.
With a sigh you turn away from the door, knowing there's no going back now. You don't know what exactly the overseer would do with you, but if it's anything like the things you imagine you're far better off venturing out into the hostile wasteland.
'Why did Dad have to run off?' you silently ask yourself not for the first time.
While the Vault was always brightly lit the cavern you find yourself in now is gloomy and full of shadows. You hear a soft rustling noise and think you've seen some movement from the shadows. Fear boils in your stomach all of a sudden and you grip the rifle so hard that your knuckles turn white.
You curse your father for leaving you behind, the overseer for being the S.O.B. that he is and the world in general for not giving you a break. Taking a deep breath you make your first cautious steps away from Vault 101.
For a microsecond you wonder where life will lead you, but then you push the thought out of your mind.
You're not in Kansas anymore. You need to start looking out for yourself and you just feel that there's something in the gloom and shadows watching you. No time for daydreaming now.
Suddenly the screen of your Pipboy3000 springs to life, casting an eerie green glow into the cavern. Your heart feels like it's jumping up your throat and unbidden memories of ghost-stories race through you mind and only with effort you succeed in suppressing them.
'Fool' you chide yourself.
The desire to glance at the Pipboy to see what brought it to life is overwhelming, but you force yourself to survey the now shadowless cavern. What you see doesn't really make you feel any better. Skeletons, and plenty of them. Some have even some rags left on them.
'Welcome to the wasteland' you mutter to yourself. 'Yay me.'
Out of the corner of your eye you glance movement and snap around, shooting at whatever it is.
With a screech the rat dies and you reload the gun.
You relax a bit. 'Just rats' you chuckle to yourself until you notice more movement. The cavern opens into a tunnel and from there you here the rustle and scratching of many small, clawed feet coming your way.
'This is really not my day' you think to yourself before sinking to your knee and starting to take aim.
There's maybe 6 or 8 rats coming your way and for a moment you think you see a bigger darker shade in the shadows beyond. You shoot and reload, shoot and reload and after the third shot and second kill you rise and start backing up cautiously. Falling over some rock or rabble right now would be really bad.
The first of the remaining rats is upon you now, literally jumping the last meter. You bring the butt of your rifle up in a purely defensive movement, more reflex then anything else, and succeed in deflecting the living projectile. With a grisly crunch you hear its spine snap as it gets thrown against the nearby wall. Two more rats are at your feet, clawing and snapping at you and you do an impromptu dance kicking at them, avoiding them and trying to stomp them into bloody smears.
With relief you note that the last two rats run off down the tunnel, scared off by the death of their brethren. Just then one rat bites you, hard, and draws your first blood. You try to shake the darn thing of you leg, but the nasty little critter has clamped it's jaws and just won't let go.
'Stupid mother..' you grumble through clenched teeth with feeling.
The other rat remains still for a moment and noticing your chance you give it a murderous kick in the side, sending it off against the wall where it drops into a motionless heap. Gripping the rifle around the barrel you whack the one attached to your leg, breaking it's neck.
With a stupefied look you realize that it's still locked onto your leg. You lay down the rifle after a quick look around and pry it's jaws open. With disgust on your face you drop the carcass. Pulling up the leg of your pants you look at the wound. It's bleeding, but only a little. Thankfully it didn't hit any blood vessels.
You rub the wound a little, trying to reduce the stinging pain and take a first deeper look at the dead rats. They're ugly little bastards, only remotely resembling the rats you know from the vault. They're also at least twice their size.
Standing up slowly, favoring your uninjured leg and securely cradling your gun you look back one last time at the entrance to the vault before heading for the tunnel and a whole new world to explore...
[edit: fixed some mistakes and spelling. I really need to pick up proof reading any of these days. Maybe tomorrow... ]