Donna turns right.
She gets a job - not posh, but it'll do, at least for now. She gets a man, eventually - one that loves her, who treats her right, who proposes. She gets married - or, at least, she attempts to get married.
Halfway down the aisle she gets abducted (abducted!) by this skinny bottle blonde of an alien, with a chavvy accent of all things, who looks... sad. Sad, and as confused to see Donna as Donna is to see her. Together, with the help of her living spaceship (a. living! spaceship!! Granddad would go bonkers if he knew), they manage to figure out why Donna got abducted, confront her lying bastard of a fiancee, and save the world from spider people.
That's right. Spider. people.
At the end, the alien - Rose, her name is, and she insists she's human - asks if Donna wants to come along. Travel the galaxy, see the universe - all of it. And it sounds grand, but... it sounds like a little much, too? Maybe she should just start with her own planet. So Donna says no, and goes back to her life.
She takes a vacation, but it's not quite the same. Too much planning, too many little details, and then when you're done, you have to go back home. Donna doesn't want that, it turns out. She wants the adventure.
So she keeps an eye peeled for adventures a little closer to home. Conspiracies, odd patterns, things like that.
Eventually, she gets another chance to see the universe with Rose, and she takes it. And it's marvelous, really it is. The people, the places, the things she's done and seen? Completely worth it, no regrets. No matter how hard it gets, sometimes.
Donna turns left.
She gets a job - not posh, but it'll do, at least for now. She gets no man, no wedding. After the first alien attack, and then the second, and the next, and the next - well. There's no time, is there? She doesn't even have a job anymore, not with London gone, and having a job's more important than a man right now. But there's more people than jobs, more people than houses, more people than anything. It's hard.
It wears her down.
And then there's the man. This strange man, a long streak of brown stripes and spiky hair, keeps popping up, wherever Donna goes. Giving her suggestions, guidance. Telling her to do things that keep her safe.
Asking her to do something that will kill her, to save the world.
Donna probably shouldn't trust him, but she does. After yet another moment of horror thanks to visitors from another world, she turns around to find him waiting, and says yes.
She travels through time (through time), meant to stop herself from turning left. Her car's not far off, but how to explain herself? How to explain what's happened, what she's doing this to prevent?
She doesn't need to explain herself, she just needs to get it done. So she gets herself hit by a car, causes a traffic jam.
He's there, after she's hit, as she's fading away. He tells Donna to take her hand, when she gets back, and tell her one thing. One word.