A/N: Thanks to grumpyjenn for the beta!
Clad in his shirtsleeves, the Doctor sat in the repair swing under the base of the time rotor. The Tardis was being sulky and he was waiting for a systems diagnostic scan to finish. It would take a practically intolerable five minutes to complete; he had already rebuilt the Orthogonal Navigation Array, polished all the thermo-coupling connector points, and now he was bored.
He turned himself in anticlockwise circles until the chains supporting the swing were completely twisted. "This is Tardis One, we are go for launch. Ignition in 3, 2, 1..." He made a rumbling noise as the chains unwound, spinning him in place like a top. "Escape velocity achieved," he giggled dizzily to himself.
There was a loud snapping noise from above.
He shrugged and began twisting the swing for his seventh go round. Probably one of his companions breaking something. They were always doing that. He released his feet and watched the tangle of overhead cabling whirl past him.
Except he wasn't traveling with anyone presently. Memory caught up with his swimming head. He had dropped off his last companion, a Hath chap called J'gorr, at least 25 years ago.
The Doctor unsteadily made his way up the stairs, only slightly bouncing off the railings in a dignified lurch. A familiar but improbable figure was leaning against the Tardis doors, watching him.
"Ah, Noble, there you are." Nodding, he braced himself on the console with his eyes closed. "Be with you in just a mo. A bit disorientated. Either I've managed to spin myself into the past, sort of Superman-flying-around-the-Earth, or something spectacularly very bad is about to happen. If I had any money, I know where I'd place my bets."
"Nice to see you too, Spaceman. I like the new Tardis desktop. The bow tie? Ehh, as long as you're happy."
"Bow ties are cool, but not effective distraction topics. Here's a better one- what are you?" Eyes still tightly squinched shut against the nausea, the Doctor put on his tweed jacket and straightened the aforementioned bow tie. He had a guest on his ship, even if it was something nasty impersonating his long-lost best friend.
"Doctor, it's Donna. Do you remember me? We... we used to travel together, a long time ago."
"I know who you're supposed to be. That's probably not a basic clone-job as I don't smell hydrocarbon polymers. And the hair, it looks too good. Genetically modifying the chromosomes for that shade of ginger is tricky. Not that I tried it on myself four times. Ok, so more advanced. You're wearing a Time Agency-issue vortex manipulator and they don't work with Gangers. Tesellecta wouldn't need it. Got to be coming off as biologically human or the Tardis would have automatically blocked materialization. Or there's the more metaphysical-y, psychic possibilities! But you're awfully benign for a manifestation of my subconscious..."
"Blimey, you can still gabble on! Guess some things never change. Hey, do you always have to be a weedy bloke with impossible hair or is that on purpose?"
"Rude! Maybe I spoke too soon." The Doctor stuck out his tongue in the direction of the voice.
Clicking boot heels marked passage up the steps to the platform, coming to rest a meter to his right. This close, he could detect the lingering ozone from unshielded rift travel. The Doctor blindly fished out his sonic screwdriver and scanned the spot.
"Stop bleepin' me," said the voice affectionately.
Feeling less vertiginous now, he risked a squinting glance at the readout.
"It's you. Actually, really you. Here. Oh, Donna," the Doctor breathed. Stepping into her personal space, he alternately peered into her face and at the sonic. "How?"
"Well, it wasn't from drinking dodgy coffee this time," she joked.
Making a choked noise, the Doctor pulled her into a hug that nearly lifted her off the ground. "I've missed you so much. Never thought I'd get to see you again. Brave, brilliant, beautiful Donna!"
He could hear her automatically draw in a sharp breath, undoubtedly to give voice to a self-disparaging comment. But instead she sighed after a moment and gently thumped his back. "Daft alien."
"I wish we had time for a proper reunion. You could tell me about your life and we'd have cakes and jam and fish custard- kindly withhold your judgment until you've tried it- and we would play table tennis with Neil DeGrasse Tyson and we'd laugh and maybe I'd cry... But you can't be here, Donna. We've got to get you back to Chiswick immediately. Every second that you're remembering your life on the Tardis is a moment that could set fire to your mind."
Donna pulled away from the hug. "About that. I'll be fine for an hour or two. The metacrisis energy is being managed for now and it bought me some time. There are some things I have to tell you."
"Managed? It's not something you can hold off like a... trip to the loo or something." The Doctor crossed his arms over his chest and regarded her seriously. "This is the consciousness of a Time Lord, fueled by the regeneration energy you absorbed. It will sweep through your mind and burn everything in its path. Think Pompeii. I have every faith in you, Donna, but this is beyond your ability to control."
"It's not me that's holding it back. God, this is going to sound bonkers... There's a third entity in my mind now. A symbiotic consciousness that can neutralise the metacrisis for a time."
"But that's wonderful, maybe I can work out a way to make it permanent! What species is the consciousness? There are some perfectly lovely symbiotic races out there that would be happy to make your red noggin their new nest. Well, some are less 'lovely' and more 'tentacley,' but I don't imagine you're squeamish. My Noble never met a calamari appetizer she didn't like." He clapped his hands and grinned at her.
Donna sighed. This was the part he wasn't going to like. "Time Lord."
The Doctor looked at her expectantly as if she had called his name.
"No, you great... I'm saying it's Time Lord. As in, two chrono-temporal presences havin' a battle royale in my brain, alright?"
The Doctor's face became suspiciously blank. "Sorry, what?"