Takes place in a not-quite-canon Doctor Who in which Jack and Donna have already met (a bit earlier than shown).

I own diddly squat. Which means nothing.

"...and then I was like, 'So, my place, or yours?' And she w-"

"-Yes, yes, Jack, that's great." The Doctor shifted the telephone to his opposite side, trapping it between his ear and shoulder as he peered under the T.A.R.D.I.S. console. "Listen, this wasn't quite the reason that I called you, I really-"

"Wait, hold on, just let me finish. So then I ask this girl where she wants to go, right, and she takes my hand and leads me-"

"Jack! Yeah - not to be rude, but I'd really quite prefer that you don't go into all of the details of your..." The Doctor trailed off and cleared his throat. "Escapade."

He gave an involuntary shudder before adding, "again."

"Alright, alright, I wasn't going to this time anyway, I know how you are with these things."

The Doctor sonic-ed a broken wire and a spark flew out. "Oi, ow." He dropped the chord.

"Anyway, Jack, I -" He frowned. "Hold on, 'these things?' What's that supposed to...?!" Pause. "Whatever. Why I called you, it's actually a bit urgent. And, when I say a bit, I mean that it's imperative that we talk right now."

There was a chuckle from the other end. "Listen, Doc. Maybe I was giving mixed signals during our trip to Sordar last week or something, but I'm really not interested in a relationship at the m-"

"Right, ye-wait, what?!" The Doctor interrupted, incredulous. "No. No, no, that's not at all what I was - no." He swallowed and rotated a screw into place. "It does, however, have to do with the Sordarian Galaxy. Or rather, the vortex that relayed us there."

"What about it?"

"It seems as though the radiation in the last couple decades has changed. There could be some... Effects."

It was quiet for a moment. "Effects," Jack echoed slowly. "What sort of effects?"

"See, the thing is I'm not exactly sure at the moment what could..." The Doctor thought for a moment. "Anyways, I need you to come here and run some tests."

Jack laughed. "I'll 'run some tests' anytime. In fact, I ran some ran some pretty good tests last night, if you know w-"

"-Jack." The Doctor rolled his eyes. "I'm being serious. I'm on my way, alright?"

It was silent.

"Jack?"

Silence.

"Hello?"

Nothing.

"Look, I'm sorry if I offended you or something, I just-" the Doctor's vision wavered suddenly.

He held his head and stood up, leaning against the console. "That was odd. Jack, are y-"

It began again. The Doctor squinted and blinked as the room swooped in and out of sight. Something wasn't right. The lights surrounding him went grey as the weary Timelord tumbled to the floor, unconscious.

"Augh..." Captain Jack Harkness rolled lazily onto his side. His head hurt and the floor was cold.

Hold on. Why was he lying on the floor? And what was that sound?

Jack's eyes shot open and he immediately reached down, only to find thin air where his gun and holster should have been. He fingered the unfamiliar fabric of the jacket he now wore for a moment before scrambling to sit up.

He was in the T.A.R.D.I.S. console room. He was dressed in the Doctor's usual attire. One of his hands clutched a sonic screwdriver.

But it wasn't his hand.

Captain Jack Harkness glanced around the room through the Doctor's eyes and chuckled to himself. "Well, this is new."

The Doctor immediately noticed that something wasn't right. First off, his biometrical composition had been radically altered, leaving him with a heavy feeling in his stomach and a bitter taste in his mouth.

Hold on, no. That was coffee. He tasted coffee. Which was strange, because the Doctor'd never developed a real taste for the Earth beverage. It always tasted a bit too... Grown-up.

Secondly, he noticed that he only had one heart. Which was a very unsettling sensation for a Timelord.

And lastly he came to the realization that he was no longer in the T.A.R.D.I.S. console room, but rather a dimly lit office. Jack's. His head rested on the table and there was a disconnected phone that appeared to have been dropped.

"That's odd..." The Doctor muttered before jolting awake. Now that definitely was not his voice.

Oh no. He looked down at his body. The Timelord was clad in a strikingly familiar pair of dark trousers with a light blue shirt. A long coat rested on a hanger nearby.

His chair shot out from under him as the Doctor frantically stood up, searching desperately for his sonic screwdriver.

Those weren't his hands. This wasn't his office.

He snatched a silver clock off of the table and peered into the glass. That was not his reflection.

"I'm Jack." The Doctor spoke incredulously using Jack's voice, stretching out his vowels to form an American accent. "I'm... Jack. I'm Captain Jack Harkness." Wow, just saying that out loud felt as if he was hitting on his own reflection.

"That's just disgusting. I'm human." He grimaced at his reflection and reached for the telephone.

"Jack?" A voice, female, sounded from the room attached to the office - Jack's bedroom. "I'm ready to go."

"Oh Rassilon," the Doctor muttered under his breath. Some girl just had to be here."Here we go."

Who wears both a belt and suspenders? The apprehensive man mused to himself as he neared the door. Can't keep his pants on, now, can he?

"Um, sorry, this is going to be a bit strange," the Doctor said, poking Jack's head into the room, "but-"

His eyes widened as he caught sight of the girl sitting on the bed.

"No." Jack's voice coughed out. "No. No, no, no. NO. NOPE. No, no, RASSILON, no."

"You-" he spluttered. "I - Jack - he... With you?! It- NO."

"Are you okay, Jack?" Jenny the Genetic Anomaly stood up, concerned. "Wait, are you British?"

This was not okay on so many levels. The Doctor took a breath and backed into the door. Really?! But wasn't she dead? And was she even of age?

"How old are you?!" The Doctor blurted.

"I'm sorry?" She raised an eyebrow. "I'm about 28. Biologically, that is. We discussed this."

The Timelord stared into space for what seemed like hours. That's an age gap of... Of... Well, that was an age gap of at least a few hundred years. Course, Jack was a bit hard to argue regarding the age category, what with his situation. But still.

"Jack? Everything alright?"

The Doctor swallowed carefully, seething. "I am going to kill him. Twice. And then I'm going to obliterate him."

"Doctor!" Donna Noble's voice sounded from the hall leading to the console room.

Jack's eyes widened. "Uh, yeah?" He called back. Shoot. He sounded too American.

Donna appeared in the doorway wearing a swimming-suit, flip-flops, and an impatient scowl. "You said we were headin' to the beach. Space Florida. Ten minutes ago."

"Uh..." What did British people even sound like? "Yah. Yas, Donnah. Let's get up and pop off and go in a tidbit. Pip pip and cheerio." Nailed it.

She entered the room. "I'm sorry?"

"Perfectly chippa, I am. Allons-y!" That was a thing the Doctor did, right?

"Yeah... I've got no idea what you're saying."

"Well," the captain started (and incidentally eminating a perfect Doctor imitation).

"There's actually sort of an issue here... It's... See... I'm Jack," he coughed out.

Donna raised an eyebrow. "You're what, now?"

"Jack. Captain Jack Harkness. Is me."

"Are you coming on to me?"

"Wh-no. What? No. It looks like there's some sort of Freaky Friday thing going on here. With me. And the Doctor." The Doctor sounded weird with an American accent.

Donna leaned against the wall. "What's this, then, some sorta weird Timelord joke?"

"I'm serious. We've gotta call the Doctor."

"Okay, 'Jack'," Donna agreed sarcastically, playing along. "Why don't we call him." She grabbed the telephone and punched in the Torchwood connection button before putting it on speakerphone and turning back to peer at Jack, eyebrows raised, head tilted.

Brrrrrng... Brrrrrrng... Brrrrrrrng.

There was a click as someone picked up.

"JACK!" There was a scream of rage from the other end. "I AM GOING TO TOSS YOU INTO - INTO - INTO THE HORIZON OF A COLLAPSING GALAXY!"

"Well, that's not a very nice way to say hello," Jack said in the Doctor's voice.

Donna glanced back and forth, nodding at the telephone. "So he's in on it, too, then."

"There's nothing to be 'in on'." Jack shook his head at her before turning back to the phone. "Listen, Doctor, not that you don't have a great body, but we need to switch back."

There was a strange sound of anger. "What, so you can continue..." The raging Timelord searched for the right word. "...canoodling with my daughter?!"

"Canoo- What are you t-" Jack frowned. "Jenny's your daughter?!" How would that even make sense? Was she a Timelord?

New species to add to the checklist, thought Jack silently, if not a bit guiltily.

"I thought you said your kids were all dead. No offense." He flinched.

"They are. Were. I mean... I thought she... I don't know!"

Donna snorted, re-establishing her presence. "Don't think I'm fooled, Doctor, your accents are rubbish."

The phone crackled. "She's alive." It sounded like a miracle. The moment passed. "Now, don't think I've forgotten, I'm still mad at you, Jack. And he's telling the truth, Donna, I am the Doctor."

"Yeah, whatever," said Donna.

"I didn't know she was your daughter!" Protested Jack (attemptfully apologetically).

"Yah." The Doctor huffed testily.

"Look, Doc, if we get out of this I promise that we won't do w- actually, probably best not to discuss that..." Jack cleared his throat awkwardly.

"So how long till you're back to normal?" Asked Donna. "Of course, figuratively. Not that I'm falling for your weird-o Timelord prank things. Just, I wanna get to the beach."

"I've done some tests and I believe the radiation should wear out in about six to twelve hours, then we'll automatically switch back. Till then, Jack... Just... Try to stay out of trouble. I'm going to go take Jenny home."

Jack grinned. "I'm going to go take a shower."

"No, you're not."

"Kidding." Probably.

-ten hours later-

"Well, Doctor," Jack said, fingers intertwined in his suspenders, "it was a pleasure to be inside your body, but it's good to be home."

The Doctor stared at him.

"That sounded better in my head."

"I should think so." The Timelord nodded slowly.

"Let's just say that we're never going to do... This... Again."

"Right. This."

"Unless we get, like, really drunk, or something," Jack continued with a chuckle. "I mean, you never know. Could be a new party game."

"Could also be a good way to get at your chums," the Doctor added jokingly. "Give 'em a tattoo, or run around doing ridiculous things in their body, that sort."

"But of course we'd never do that."

"Oh, yeah," the Doctor agreed. "'Course."

They nodded and went their separate ways, the Doctor smirking to himself.

Jack wouldn't notice the butterfly tattoo reading "I am an insensitive prick" on his lower back till much later.

Thanks for reading! Be sure to leave your thoughts in that lil rectangle below ;)

- Impossible Oswin