Author's Notes: Written for who_contest One Shot Challenge #12: 'Gravity'.
Having thousands of complex thoughts occurring all at once in the confines of a human mind is, she decides, a lot like free-falling. It's terrifying to feel as if she's spiralling endlessly through space, but it's also far more exhilarating than anything she's ever experienced before. That's saying a lot, too, considering some of the mad things she's witnessed since meeting the Doctor.
At any given second, the full collection of everything the Doctor knows is right there at her fingertips. All that knowledge rushes past her in a seemingly endless stream as she plummets through the newly accessible expanses of her own mind.
It's amazing to see the universe through a mixture of the Doctor's eyes and her own humanity. Even the simplest things seem extraordinary.
Perhaps most importantly, she can finally really see herself the way the Doctor sees her. For a while she's fully conscious of exactly how brilliant she is.
But only for a while.
As much as it might feel like flying, she's well aware how every moment brings her closer to the ground. She's falling fast, and there's no such thing as a bottomless pit. She knows that right now the Doctor would undoubtedly spout some silly quote about the light burning twice as bright and half as long, because that's what she's thinking herself with his thought patterns influencing her own. She'd roll her eyes at him, but that wouldn't stop him from being right. This will end, and soon at that. Gravity will catch up with her, as it always eventually must. And she knows she surely can't survive the fall from such a great height.
But although she personally decides it might be worth living only a few more hours to able to experience this for the rest of her life, the Doctor refuses to let her make that choice. Even now that they're mental equals, he still thinks he knows better than her.
He manages to pry himself away from her despite how desperately she's holding on. In the blink of an eye she loses more than just the ability to share his intelligence. She loses him completely, and part of herself with him.
The impact of hitting the ground is just as fatal as she'd predicted.
Donna Noble will simply never remember that she died.