He had dared to hope, just for a little while, that she could stay with him.
Donna Noble was special, after all. He had sensed it from the beginning, when she materialized on board his shielded TARDIS in full flight; and that had been the least of the amazing things she'd done since. Of course he'd had to dismiss her from his life for a while, after she'd refused his invitation; he'd allowed Martha to take the place that would have been hers, and told himself he'd made a mistake. But when Donna came barging back into his life and planted herself in his TARDIS, dragging him out of his self-pity by the scruff of his neck and bullying him back into compassion–it hadn't been long before he'd remembered.
Rose had indulged his old self and all but worshipped his new one; he'd loved her, but all the while he'd known she couldn't see his faults clearly, or he hers. They'd shared a delirious blindness for a while, but it could never have lasted. Martha had fallen in love with his lips on hers and the frail hope that one day he might come to mean that kiss instead of disowning it; she'd been magnificent, but for all her worthiness and all his regrets, she'd been wrong. Donna, though–she'd mocked his body, scorned his face, and in almost the same breath she'd sworn to stay with him forever. It wasn't the shell of him she loved, not the part of him that changed with every regeneration and would one day change again; it was the part of him that he himself had always valued most–his mind.
And now she had it, and it was killing her.
He wasn't in love with Donna, any more than she was with him. She'd kissed him once, slap on the mouth with a tingling aftershock like Russian vodka, and it hadn't changed either of them. But she knew him, understood him, as deeply as any companion he'd ever known. She'd bellowed sense into him when his thoughts were in chaos, stood up to him fearlessly while others cringed and fled, broken his hearts with her unexpected tenderness and healed them again with a bray of her ridiculous laughter. Strong as a lioness when her ideals were threatened, yet for all that her self-worth was brittle as mica; she'd never been able to see how amazing she was in her own right, and now–the thought wrenched at him, stinging tears to the surface of his eyes–perhaps she never would.
He'd known this moment might come, dreaded it as soon as Donna had popped up behind Davros's console and he'd seen that alien light in her eyes–his own reflection gleaming back at him, all smug superiority and manic brilliance. And yet she'd been through so much with him, come so far, and she seemed to be handling the transformation so beautifully, that he'd allowed himself to believe she might, by some miracle, survive. That her generous, elastic mind could expand still further, take in the full scope of his universe and hold it indefinitely without breaking–it had been the tiniest ember of hope, but he had cupped it in his mind, refusing to let it die.
Especially when he'd seen how calm she was, how poised in her new Time Lord confidence; still bold and full of laughter, but with an ageless serenity beneath. It had suited her so well, and the fierce joy of working side by side with her as an equal had been so great, he had nearly persuaded himself that there would be no need for him to intervene, or even to warn her of the danger. Surely her mind would have crumbled much earlier, if it were going to crumble at all?
But then all the others had gone, leaving just the two of them alone in the TARDIS, and as he'd walked up the ramp toward her he'd seen that first hectic spark fire behind her eyes, dread harbinger of her inevitable breakdown. She was talking too fast, her movements full of fevered energy–the symptoms were visible now, and he could not deny it any longer.
She understood what it meant, what was happening; the knowledge and the anguish were there in her face. And yet even as they spoke he could see her drowning, her panic strangling all that elevated Time Lord reason and even her native common sense, leaving her agonizingly helpless before him: she had become a wounded animal, ready to die in the trap that held her rather than allow his hands to quiet her and set her free.
But Donna Noble, the Donna he knew and loved, would not want to die.
I'm so sorry.
Her eyes were wide and red-rimmed, her lips whispering desperate syllables, pleading for a miracle–but it was too late. He framed her face with his palms, his fingers pressing her temples with a lover's gentleness, and in her mind he kissed each memory of the time they had shared, as he took them all away.