Disclaimer: And this one isn't mine, either. Maybe the next episode.
AN: I WIN.
Donna is aware there is something wrong with the world. Oh, aliens invading and all that is wrong enough, but she knows it's something else. Just one tiny little thing that she can't quite put her finger on.
It's like forgetting someone's name when you see them again, even though you know you know them. It's right at the tip of your tongue, but it just won't come out.
But it isn't a name this time. Or maybe it is. She just knows that something doesn't fit. And it's not the looks everyone keeps giving her. It's the inexplicable feeling that something is wrong in her life.
She's sure that if she can only figure out what then everything will be okay. This feeling lasts until a replica of the Titanic crashes into London and she watches the mushroom cloud blossom in the distance; safe at the inn with mum and granddad.
Afterwards, she doesn't really think about it much. There's so many things wrong with the world now that one small thing rather gets lost in the shuffle.
Ultimately, standing on that sidewalk watching the truck barrelling down the road, she's willing to make the sacrifice, because she knows without one single doubt that that little thing that she thought wrong with the world is something that she could have changed all along. It's her own fault. And if sacrificing one life for another will save the universe, well it's better than anything else she's ever done.
She's not a disappointment now.