The red-haired woman striding from the TARDIS was not Amy or Donna or anyone else Jack recognized. Someone from his future, maybe, and the Doctor had gotten the timeline wrong? From the purpose in the woman's gait and the flashing steel in her eyes, Jack guessed he and the rest of the Torchwood office would know soon enough. Then she snapped her fingers and the TARDIS doors closed behind her, with no sign of any other occupant, and Jack revised his opinion of her identity.
"Finally ginger," he guessed, "but you have bigger things on your mind right now than hair color." He let his gaze stray down to the Doctor's chest, where there were indeed bigger things to worry about.
"You," the Doctor snapped. "Sit."
Jack, taken aback, sat down on the edge of the nearest work station. He'd expected a reaction to his comment, probably an indignant one, but straight-up anger was troubling. "Did I do something wrong?"
"Yes. No. No, not intentionally, but it's still your fault."
"Don't play the fool, Jack. I'm supposed to be a man! I'm always a man! Don't get me wrong, I love women, women are fantastic–and I still can't use that word, the teeth are all wrong–but they're not me! I'm not one of them! And now look at me!"
Jack refrained from saying out loud "Trust me, I'm looking," but it was a close thing. "What's this got to do with me?" he asked instead.
"Like you don't know."
"I don't," Jack answered, honestly baffled.
"Oh." The Doctor tugged a strand of her hair and cleared her throat, giving Jack a moment to contemplate the jarring pronoun change. "Ehm. I mean to say, shouldn't we discuss this somewhere else?"
Jack looked around the office, at all the curious heads peeping out from behind doorframes and cubicle corners. "Yeah," he agreed. "My office?"
Jack followed the Doctor into the blue box, bopping the head of the first agent he passed as a reminder to get back to work. Inside, the Doctor leaned against the console and crossed her arms, facing Jack.
"You ever watch Jurassic Park?" she asked.
Jack crossed his own arms and leaned against the railing, mimicking the Doctor's posture. "Based on the book by Michael Crichton," he said cautiously. "Sure."
"The dinosaur population is controlled by genetic manipulation. All the dinosaurs are female. But they use the DNA of a certain frog to help reconstruct the missing pieces, and–"
"The frog DNA lets the dinos gender-swap. I remember."
"It's an evolutionary adaptation to limited reproductive opportunity."
"Are you telling me Time Lords do the same thing?" That didn't add up. Somehow Time Lords and hermaphroditic frogs couldn't quite fit comfortably in the same thought, at least in Jack's mind.
The Doctor pursed her lips. "Not exactly. Regeneration is a bit like throwing darts at a wall, blindfolded. You can't control what you hit, but the general direction is determined by which way you're facing. Subconscious impulse plays a big role."
"Let me make sure I'm following." Jack felt a smile creep onto his face. "You regenerated as a woman because somewhere along the line, you've developed a subconscious impulse to save your species by having children?"
"I don't see why that should be funny. It's pure evolutionary biology."
"Okay, okay." Jack schooled his face and voice to neutral. "But in all seriousness, this is the fourth time you've regenerated since the War. Why now?"
The Doctor cleared her throat again, suddenly avoiding Jack's eye. "Well. If you must know. There are certain... conditions, under which this sort of thing makes sense. For instance."
Jack waited. The Doctor tugged on her hair again. Apparently that was a new nervous habit. She'd done it once already, when Jack had asked what the Doctor's sex change had to do with him... oh, that...
He found he didn't have the courage to voice his theory just yet. "Hang on, I'm still confused. The dinosaurs in Jurassic Park only switch to male because there are already plenty of females around. If you're the only Time Lord, it shouldn't make a difference which sex you are, because in either case you've got no one to mate with. Right?"
"Like I said." The Doctor kept her gaze fixed off to Jack's right and her jaw so tight she spoke through clenched teeth. "Conditions."
And there was the confirmation Jack needed. "So last Christmas, when you lost the TARDIS in that snowstorm and you and I–"
"I remember last Christmas, Jack, thank you for bringing it up."
"I should hope you do." He gave up trying to contain his amusement and promply grinned so wide his cheeks hurt. "So it really is my fault. Hang on, does that mean I was the first person since the War to–"
"No, it does not mean that! It... means you might've been the first male, that's all it means!"
Jack bit back a guffaw with herculean effort, because he suspected that if he laughed outright, he would be banned from the TARDIS for the forseeable future. "Right," he said, his voice coming out only slightly strained. "I have one more question. If that's okay."
"Choose it wisely, then." The Doctor seemed to flip a switch, her previous embarassment gone. She looked him in the eye with all the foreboding intensity Jack normally associated with the Time Lord. Time Lady. He thought for a few moments before responding.
"Will it do any good? I mean, you're still the only Time Lord, female or otherwise."
"Prenatal environment is crucial for a developing Time Lord. Almost more crucial than genetics." The Doctor fiddled idly with part of the console. Jack suspected that like any other regeneration, this one was most comfortable in lecture mode. "Melody Pond had a hundred-percent human genome, and look how she turned out. Hmm, maybe I ought to slap a warning on this old box. 'Do not ride if you are pregnant or attempting to become pregnant.' Or have a heart condition. Unless you want a baby with a time-head. But like I was saying, if–and this is purely hypothetical, mind–if I carried a child to term and spent some time in the Vortex during crucial developmental stages, then for all practical purposes it wouldn't matter what species contributed the other half of the child's DNA. Beyond certain limitations regarding compatibility, naturally. Anything bipedal, mammalian–"
"Devilishly handsome," Jack put in.
"Jack Harkness, if you cannot treat this subject with the gravity it deserves, you will not be fathering any Time Tots, is that clear?"
"Yes." Jack sobered instantly, if sobered meant feeling like he'd been hit simultaneously with a bucket of ice water and a live scorpion. "Yes, I'm gravity. I mean, I'm serious. Incredibly serious. You've never seen me this serious. What's that you said about fathering?"
The Doctor's mouth twitched like she was trying not to laugh.