"Utilitarianism," Donna announced once the TARDIS was moving again. "Jeremy Bentham, John Stuart Mill, The Doctor... Acting for 'the greatest happiness of the greatest number'. The bigger picture, the greater good. I understand it all now, Doctor. Whatever pain it causes, to you in particular, you have the obligation... the inclination towards utilitarianism. I see it now."
The Doctor did not move from his moping position against one of the coral pillars set back from the console. His face was set to a frown already, but his eyes saddened more by the second as Donna spoke.
"And why do you see that now?" he asked, darkly.
"I see everything! All that was, all that could be... Biggest brain in the universe! Well, present company excepted... I was thinking we could go to Thelspoon next. The planet Thelspoon, what a great name! Apparently they have mountains that sway in the breeze... MOUNTAINS that move!"
"And how do you know that?" The Doctor was terrified for Donna's fate, now that all other horrid goodbyes were out of the way.
"Cos it's in your head! And if it's in your head, it's in my head!"
"How does that feel?"
"Brilliant, fantastic, Molto bene!" Donna exclaimed as she danced over to the other side of the console to press some buttons. "Great big universe crammed inside my head! It's great! How do you get anything done? Oh, you don't! You know, you could fix that chameleon circuit if you just tried hotbinding the fragment links and superseding the binary... binary binary binary binary binary binary binary binary..." She shook quickly, head to toe, breaking the loop, and smiled. "I'm fine!"
"Donna..." The Doctor now looked crushed, witnessing the short-circuiting of Donna's oh-so-human brain. He walked over to her, the emptiness within him ever growing as he slowly came to terms with what happened next.
"What I don't understand though, Doctor, is how you can do it? I understand the premise, the ideology, but to actually carry out these plans that shatter you so much... I couldn't do it."
"That's because you're human. Or half-human, whatever! The human brain is just too tiny and limited to comprehend the vagaries of time and space and universal balance!"
"Here we go again, insulting species! I know you're really upset when you start picking on humans. Bully-eeeeeeeeeeeeeeeeeeeeeeeee...e-e-e-e-e-e!"
"Donna..."
"It's nothing, I'm ok!" She turned away from him, leaning on the TARDIS console and gazing distantly at the monitor before her.
"Donna, listen to me. You know what's happening now?" Donna nodded shakily. The Doctor leant in closer to her. "This has never happened before. A Time Lord-human metacrysis. And you know why..."
"Because... it can't," Donna conceded slowly, as she turned her eyes to meet his once more. Tears began to form in hers, and The Doctor had to fight hard not to follow suit.
"Donna, you know what I'm going to have to do..." whispered The Doctor, as reluctant as she to do what he was thinking.
"No," Donna protested. "Doctor, please." She pulled away from him. "I'm not going home, please no!"
"I'm sorry. I'm so sorry..." The Doctor began, following her across the room.
"But there's got to be another way!" Donna was desperate. "Oh, I know! What about that Chameleon Arch thing? Martha told me about it!"
"Donna. Engage that brain of mine. The Arch only works on Time Lords; it'd change half of you, but then you'd just be even more scrambled. Resulting in-"
"My death," Donna interrupted, accepting the blow with utmost nonchalance. "Ok, so that's out... can't you just wipe the Time Lord part?!"
"I wish I could but..."
"Well what then Doctor?! You bloody lump, surely there's something you can do!" In her frustration as she span away, Donna struck out and sent a discarded glass flying. The Doctor caught up with her and gripped her arms.
"Yes, there is," he started, eyes boring into hers, almost matching them in wetness. "I can clear your memory of all of this, of me, and take you home."
"I'd rather die!" Donna snapped, looking away from his devastated brow.
"Donna!" The Doctor was astonished and slightly disgusted. He shook her firmly, forcing her to look him in the eye. "Donna Noble, don't you dare say that! Don't you even think it! You've got so much to live for, so much to do out there! I just wish you could see... You're special, you're brilliant, and you can do whatever you want!"
"Yeah," Donna said softly. "But I won't will I? I'll go back to being nobody, to not caring! It'll be like I never met you, nothing changed. It's unbearable to even think..." She trailed off as tears streamed down her face. The pain she felt scorched The Doctor equally. It was true, much as he hated to admit it. She'd never have realised her brilliance if not for him. It even took him all this time to make her see. Without any of the memories or the thoughts, she'd go back to being unconfident, uninspired, and disbelieving.
"There might be something I can do," he muttered. Donna stared up at him, her eyes gleaming with hope. "You'll still have to go back..." Her face dropped once more. "I'm sorry, but the memory or knowledge of any of this would blow your mind. Literally! And that can be messy... I remember this one time, back when I was such a young thing..."
"Doctor!"
"Right. Sorry. You see, I can leave you with something. It'll be hard to tune and a lot will rely on you but... I can give you a sort of residual memory..."
"Of you?"
"Oh no, far too dangerous! I'll just... I can leave you with... a desire..."
"You what? This better not be dodgy!!"
"Now that's more like my Donna! No, I can make sure that when a trigger is given, you realise a new urge to find a greater potential." He noted her scepticism. "Give me a chance! I can give you the inspiration to go out and seek bigger and better things. No offence Donna, but you're wasted as a typist! Look, you won't know where it's come from, but you'll know. You'll know you want more, and you know what? You'll go and get it."
Donna looked on at him, still unsure. However, her breathing was becoming quicker and shallower. They both knew time was at a premium.
"Donna, it makes sense. Otherwise it's all or nothing. And I'm not going to let you die. You're better than that."
Donna slowly nodded.
"Make it quick," she stammered. The Doctor smiled darkly. "Wait, Doctor. Before I ... go... I just want to say thank you. For giving me all this, even if I don't know about it come morning."
"No... Thank you. You saved the universe, and I think you taught me more than I you." They shared an ironic chuckle before The Doctor pulled her into an embrace. Donna cried freely now. "Oh Donna... We had the best of times, we really did." She nodded into his shoulder. "You were brilliant," he went on. "I'm so sorry." He parted from her and raised his hands to her temples.
"Doctor?" she said finally, halting the final push momentarily. "Look after yourself, yeah? For me?"
He gave his first real smile since Cardiff. "For the DoctorDonna," he said, as his fingers met with the sides of her head, and the great minds melded.
- - -
"So he's just left them there?!" Martha exclaimed in disbelief as Jack explained to his new comrades about The Doctor's latest sacrifice. Jack nodded grimly, eyes set ahead of him on the route to The Hub.
Mickey let out a low whistle. "Rose won't have taken kindly to that!"
"She'll get used to it easy enough," Jack countered, shocking himself with his flippancy.
"And what about The Doctor?!" Martha protested. "The real one!" Silence reigned for a time whilst they all mulled it over with varying levels of pain. "We have to go back!"
"What?" Mickey said, simply.
"No," Jack said, even more simply.
"What, but he'll need us! He's going to be shattered, Jack!"
"That's possible, yeah," Jack said with difficulty. "But he has Donna. We'd only get in the way. Leave it to her, she knows him enough now to sort it out. We discussed. He knows where we are when he does need us... for now, there's Donna."