Disclaimer: I don't own Doctor Who.
A/N: For the spoiler_song ficathon at LJ. The prompt was 'River. She always keeps her promises, in the end.'
River Song may interpret promises her own way, and rationalise them leisurely, but she never breaks a promise to the Doctor. (Sometimes she pretends to, to honour a promise she's made (a) him at another point in time, but that's something else entirely). She simply never makes him a promise she can't keep. She's agreed to terms to that extent.
He grabs her hand on a cold night under cold stars and says, "If there's…" He trails off, bites his lip and pauses for the longest moment. She's sure he's made himself stop, but then it comes, "If one day, something happens, and there's any way… if there is the slightest possibility, the tiniest glimpse of a chance… Promise me you'll save yourself over me. Promise me."
His sincerity chafes in her chest and ignites her intuition, and anything that can make him grimace like that and use that tone of voice must be important. And what's important to the Doctor...
Her throat is a bit dry. "I promise… that I'll try."
His hand convulses around hers and her heart skips a beat in response. He doesn't say anything more, just nods once and then hangs his head as if prematurely defeated. Anything that can make him behave like that is also likely to be something utterly stupid, she thinks. So she promises herself she won't let him do anything stupid, not even if it means she chooses him over herself.
When the time comes she's so glad she technically only promised him to try, because it is so very stupid, so very stupid, and she could have saved herself; there's always a way and a TARDIS, but she doesn't, because it's completely impossible, and the fact that he even asked it of her makes her irrationally angry.
That technicality means she doesn't break the promise she made him that night, and that is all kinds of important because it gives her a perfect score (if she had a second to spare she would send a message to the Doctor, to her Doctor, the one who really knows her. Promises broken: 0, his psychic paper would say, and he would understand). As for what she promised herself… Well, River Song never breaks a promise to River Song.