Summary: When the Fourth Doctor regenerates into his Fifth incarnation, he runs into problems…only not the problems shown at the end of "Logopolis", all thanks to the Time Lords' ancient enemies, the Giant Vampires. As a result, The Doctor and Tegan are unexpectedly forced into a symbiotic relationship. Could it deepen into something more? And how will this unexpected complication figure into the Master's evil plans?

I own nothing except the plot. But oh what I'd do if I did own "Dr. Who"...

To my loyal readers who know I've got other stories that need to be finished: Apologies. This plot bunny has literally overtaken my entire life. I actually have 100 pages of this story written and am adding to it daily. I plan to update this once a week and hope that my muse lets go of my brain long enough for me to finish my other 3 outstanding stories before, say the next millenium. Thanks for reading and I hope you all enjoy and let me know you do by (of course) leaving lots and lots of lovely reviews!

Part One: The Enemy Within

The TARDIS in front of them, out of sight but not so far it can't be reached on foot. Adric, lying dead or unconscious on the ground behind them next to the equally dead-or-not Pharos Project guards. The Master's TARDIS practically on top of them, shut tight with the Master still inside with who-knew-what intentions. (Hint: Nothing good.)

The Doctor running, jogging, Nyssa by his side, Tegan in his arms, unconscious. Bleeding from some sort of throat wound. The two of them literally glowing with regenerative energy.

Nyssa, desperate with fear and worry for her two friends, one old, one newly met, tugged on the Doctor's arm. "But what about Adric? We can't just leave him—"

"We have to." The Doctor never even broke stride, never turned to look at her or back toward their fallen comrade. "If he's dead, there's nothing we can do for him. If he's unconscious, then the Master will keep him alive—most likely to try and get to me." His gaze darted down, lingering on Tegan's face before he returned his attention to not tripping on his oversized scarf on the uneven terrain, and Nyssa saw the conflict there, along with—could that possibly be guilt? "Either way, if we don't get back to the TARDIS so her shielding can help stabilize us, Tegan and I will both die."

One Hour Earlier

Regeneration. Nyssa had heard of it, the Doctor had even deigned to explain it to her once. As a scientist with specialties in biology and biomechanics, she found the entire process fascinating. The idea of a humanoid life form that could literally rebuild itself after fatal trauma, cell by cell, into another living being with the original memories and something of the original personality intact in the rebuilt brain was as astounding a concept as she'd ever considered.

But nothing the Doctor had told her, none of the research or information he'd allowed her access to, had prepared her for the unnerving situation that was unfolding right before her horrified eyes.

It started with a fall. A simple death, almost anticlimactic after everything they'd gone through to reach this moment in time. A moment, according to the Doctor, that had been "prepared for." The "friend" that had rescued her from Traken shortly before the Master's plans destroyed it; the half-seen Watcher who seemed to haunt their movements on Earth and in Logopolis; both revealed to be one and the same person.

And that person was the Doctor.

How his future self had managed to cross his own timeline in order to help them she wasn't sure; why he didn't appear fully in his regenerated form she had no idea. The white, almost ghostly figure was one she'd seen no references to in the regeneration database the Doctor had given her access to. In fact, she didn't recall reading anything about the moment of a Time Lord's imminent regeneration being "prepared for" in any way at all, let alone by manifestations of the immediate successor aiding and abetting in the matter.

The initial process had seemed almost…peaceful. The Doctor lay on the ground, his body broken and twisted by the fall from the Pharos Project antenna, but his gentle humor still clear in his eyes as he assured his three companions—herself, Tegan Jovanka of Earth and Adric of Alzarius—that all would be well. "The moment has been prepared for," he'd murmured.

Then his gaze had been inexorably drawn to that of the distant figure of the Watcher. Not so distant now; as it reached his side, the form began to lose substance, merging into that of the now-unconscious Doctor, overlaying the substantial image with the ghostly, until they became one and the Doctor's new form was born.

Things had gone rapidly downhill from there.

Instead of the golden burst of energy that Nyssa's research had led her to expect, the Doctor simply shimmered for a moment; watching his two forms, future and present, merge was almost like a seeing a double image transpiring from a blow to the head. At least, that was how Nyssa felt about it. She wasn't to know how the others felt as two things happened very rapidly after that: first, the Pharos Project guards arrived, waving guns and demanding to know what was going on. And second, the Doctor let out a horrific cry of pain as the golden storm of regenerative energy finally burst around his body.

His, and that of Tegan, who was still kneeling by his side after Nyssa and Adric had risen to their feet to try and fend off the approaching guards.

The sudden burst of energy, the blinding golden light, both had stopped the guards in their tracks, but only for a moment. Even as the energy continued to build and grow, even as Tegan's screams joined those tearing from the Doctor's throat, they moved forward. More cautiously, but still forward. Toward them. One of them had raised a small, black rectangle to his mouth, undoubtedly a communications device of some kind—seeking assistance, Nyssa assumed, but for themselves or for the couple who appeared to be burning to death in front of their very eyes?

Either way, Nyssa knew they had no time to waste in dealing with these Humans. They would no doubt treat the time travelers as intruders first, victims in need of medical assistance second. And they would probably try to detain Adric and herself for questioning of the sort neither could give satisfactory answers to.

Fortunately she and Adric were of one mind on this matter. Unfortunately, the Master chose that exact moment to once again wreak havoc among them.

Adric had moved toward the guards, hands upraised, spouting some nonsense about their group being alien intelligences (true enough for three out of the four of them) that had come in answer to the Pharos Project's call (out-and-out lie), although two of their members were currently undergoing "difficulties" (incredible understatement).

"Difficulties" indeed. Difficulties that were exacerbated when the Master's TARDIS materialized, emitting bolts of energy that rendered Adric and the guards unconscious. Or possibly dead. Nyssa had no time to examine them.

Because while she'd had been distracted by the unwelcome chaos, the Doctor's regeneration had apparently completed, since the next thing she knew he—the new "he"—was standing next to her, holding an unconscious—and bleeding—Tegan Jovanka in his arms, urging Nyssa to run, telling her that Tegan's life was at stake and they had no time to spare even for Adric's sake.

Which was where things now stood; Adric possibly in the hands of the Master, he and the Pharos Project guards either dead or unconscious, and she and the Doctor frantically trying to save Tegan's life.

The Present

"Nyssa! Hurry, we have to get her to my quarters now!"

Nyssa shook her head to clear away the distracting memories of what had just happened as they stumbled into the TARDIS. He said Tegan was in danger, and she had no doubt about that, having heard the other woman's cries of pain, seeing how pale and drained she looked. She was obviously in shock, no doubt due to the regenerative energy still coursing through her system, but what could have caused the still-bleeding injuries to her neck? It was hard to see them in any detail, but the single glimpse she'd had before the Doctor outpaced her seemed to show a pair of precisely placed puncture wounds directly over the other woman's jugular. Curious and unsettling, but surely the Doctor needed medical detachment and not a fretful companion right now.

Still, she couldn't help asking her next question, since this clearly was some kind of medical emergency. "Shouldn't we bring her to the medical bay?" She'd sped up to match the Doctor's long strides as he dashed down the corridor in a direction she'd never taken before. Presumably toward his private quarters, wherever they might be located in the maze that comprised the TARDIS interior.

"No," he said over his shoulder. Just that. "No." Nyssa was left to grope after a possible explanation as they skidded around another corner. Did he have some sort of equipment in his private quarters designed to deal explicitly with non-Time Lords who were undergoing regenerative trauma? Perhaps this sort of thing had happened before, even if the details had never made it into the database she'd been perusing.

Why hadn't it? She pondered the question as they sped along, concluding that it just wasn't the sort of thing one put into a database meant for public consumption. She hurried after the Doctor. She didn't bother asking him; he didn't seem inclined to conversation at the moment. Nor should he, she chided herself as they careened around another corner and started down a slanting access ramp that presumably led to the next level down. After all, the Doctor had more important things to deal with at the moment.

Such as saving Tegan's life.

Just as she reminded herself of that grim fact, the Doctor came to an abrupt stop. The ramp they'd been descending had leveled out to a short length of dimly lit corridor—wait, were those torches set into the wall? She gawked a bit at the completely unexpected sight; why in the world would the Doctor use such an archaic—not to mention hazardous—means of lighting anywhere on the TARDIS? It made absolutely no sense.

It didn't help her growing unease as she realized that he, too, seemed put off by the sight of the open flames; he hesitated a moment longer before appearing to shrug it off as he hurried to the door that marked the end of the corridor.

A door that was as archaic, as out of place, as the torches in their black metal sconces.

It was wooden, a dark, smoke-stained wood covered in elaborate carvings and banded on either side with rows of black metal triangles that were stylistically similar to the torch sconces. Even the handle appeared to be made of the same metal

The Doctor hesitated again when they reached it, frowning as if it wasn't what he'd expected to see. Nyssa, stumbling to a stop behind him when he ceased moving, craned her neck to peer past him, then looked up inquiringly. "Did we make a wrong turn? Is this the wrong door?"

"No, it's the correct door," the Doctor responded. His new voice was slightly higher in pitch but otherwise little changed. Another aspect of his regeneration she'd have to get used to. "I believe the TARDIS is just having a little joke at my expense, that's all." He scowled up at the ceiling. "Lovely, I can't wait to see what else you've done with the interior decorating."

Without explaining his curious statements, he shifted Tegan in his arms so that one hand was free. "Shall I open it?" Nyssa asked, stepping forward and reaching out to do so, but he shook his head.

"No. You can't. Isomorphic," was his curt response as he reached out and placed his hand on the handle.

The door swung open at his touch and he shouldered his way into the room. "Hurry, Nyssa, we have to get her out of her clothes!"

She'd come to a dead stop at the threshold; how had the Doctor known to expect his quarters had been altered in their absence from the TARDIS? Because this couldn't possibly be a bedroom he'd chosen for himself, it was far too..too…too much. Too dark, too intense…had he made a mistake after all, were they in the wrong room?

"Nyssa!"

She snapped out of her temporary paralysis to see the Doctor in the process of laying Tegan gently onto the crimson coverlet of the massive, four-poster bed that dominated the room. A room dimly lit only by candle- and fire-light. And a bed so high that it had to be reached by ascending a set of steps on either side…she shook her head and hurried over, hastening up the three steps opposite the ones the Doctor was occupying. She suspected her mind was dwelling on trivialities as a way to avoid thinking about the Doctor's cryptic insistence that Tegan needed to be here, in what he'd definitely identified as his quarters, rather than in the medical bay. Certainly there was no sign of any equipment that might be needed, although there was always the possibility that it was something as easy to tuck away in a drawer as his sonic screwdriver.

He hadn't bothered turning down the counterpane, just placed Tegan on top of it—and was that some kind of animal skin thrown across the foot of the bed? Suddenly his mutterings about what the TARDIS might have done to his private quarters made a little more sense. It was manifestly clear to her that it must have reconfigured the Doctor's chambers upon his regeneration—although how it had known and why it had done so remained a mystery.

A mystery among many mysteries. "Doctor, why do we have to…" Nyssa's question trailed off as she watched, bemused, while the Doctor began tearing his own clothing off, quite literally ripping his shirt from his body, buttons popping, seams tearing, the oversized scarf thrown willy-nilly to the floor along with hat, jacket, trousers and the rest. Her hand automatically continued to undo Tegan's jacket as she watched, open-mouthed, while the Doctor stripped down to his underclothing—which she averted her eyes just quickly enough to avoid seeing him step out of.

When she looked up again, blushing furiously at her first glimpse of a naked man, he'd jumped up onto the bed and shoved her hands aside.

"Too slow," he muttered. "This is taking too long, we need closer physical contact…Nyssa, be a good girl and go to the nearest pantry—there's one quite close by, make a left when you get to the top of the ramp instead of a right, go about fifty meters or so and you'll find it—and bring back foods that are rich in protein. Also, juice. A large quantity of fruit juice, if you would. She'll need them both when she wakes up."

"Doctor, you have to tell me what you're doing; none of this makes any sense!" she protested, continuing to avert her eyes as the Doctor removed Tegan's clothes almost as rapidly as he'd removed his own—and with as little concern for their future wearability.

"Tegan's undergoing an ancillary regenerative transformation," he snapped. "And no, I don't have time for more detailed explanations, Nyssa. You're just going to have to trust me. Can you do that?"

"What about Adric?" she asked, not quite ready to back down. The Doctor's feverish impatience, his obvious concern for Tegan's health, was the only thing keeping her from stubbornly refusing to do anything else without a detailed explanation as to what, exactly, was going on. Because "ancillary regenerative transformation" meant nothing to her.

"After you bring the supplies, it might be safe enough to do a bit of a reconnoiter, see what you can find out." He still sounded distracted, as if his mind wasn't fully on her question, but he suddenly turned and looked directly at her for the first time since they'd entered the TARDIS. The obvious worry in his eyes did much to quell her growing unease. "We'll save him, Nyssa," he said gently, "Never fear. And yes, it would help if you could see what you can find out. Just try not to let yourself be seen if you can help it—and if you can't, you should still be safe; the Master already has a hostage, he'll have no need to take you as well."

As swiftly as he'd granted her his undivided attention, it was gone. His gaze drawn inexorably back to Tegan, he leaned over her unconscious form, examining the punctures in her neck with a black frown marring his face. "Hm, perhaps it isn't as bad as I…are you still here?" he snapped, suddenly raising his head to glare at Nyssa. "I told you, she'll need the food and the juice when she wakes up, which should be soon." His voice turned uncertain as he added: "I think. Maybe. At any rate, there's nothing more you can do for her and I know it'll set both our minds at ease if you can at least discover if Adric is still alive."

Nyssa found herself nodding agreement even as her mind shied away from the thought that her friend might not have survived his encounter with the Master. It was more likely he'd been taken prisoner, and equally likely that the Master was still here on Earth, working out his next evil scheme.

The thought of being instrumental in foiling any such plots was an enormous incentive; even if Adric weren't involved, anything she could do to stop the man who'd single-handedly destroyed her home world and murdered her father was well worth the risk to her own life.

She'd still prefer to have the Doctor with her, but if he needed to be with Tegan right now, if there was some really good explanation for why he was pulling her into his embrace and nestling his naked body next to hers, holding her very closely against him—aside from the obvious, which was completely inappropriate and clearly not his intent—then Nyssa would do as he asked.

oOo

As Nyssa finally nodded and headed out of the bedroom at a near-run, the Doctor allowed himself to relax, just the tiniest bit. If he left Tegan, even for the short time it would take to fetch the items he'd requested, the results could be disastrous for both of them.

Nyssa didn't understand, and how could she? He barely understood it himself. In four lifetimes, he'd never faced this particular Time Lord blight. None of his immediate family, what was left of them, had; none of his friends and classmates had either, not even the Master. Of all of them, he thought bitterly, it would've been the Master who'd have embraced the taint. But no, he'd never shown any signs that the contamination had affected him in any regeneration, and as a purely biological manifestation it wouldn't have survived him stealing Tremas' body.

His lip pulled back in a snarl as he thought of his once-friend and now greatest enemy. The Master had much to answer for, and if Adric had suffered anything more than simple unconsciousness, there would be yet another item on the Master's ever-increasing list of crimes requiring retribution. Still, there was nothing he could do about it, not right now. Not until Tegan had stabilized, not until the regenerative energies had run their course through both their bodies. Even so, it felt like sacrificing one companion for the sake of another as well as for his own, selfish needs—but either Adric was alive and therefore able to be rescued, or he was dead and there was nothing that could be done about it.

Except, of course, to avenge him when things had calmed down a bit.

Nyssa interrupted his brooding thoughts just long enough to drop a heavily laden oval tray onto the foot of the bed—carefully avoiding the wolf skin, the Doctor noted with one part of his mind. Then she turned without a word and left the room, leaving the door slightly ajar since she wouldn't be able to open it again. Good girl; he knew he could count on her to act first and save the rest of her very understandable questions for later.

The small smile that comforting thought brought him faded into a frown as he considered her situation. It was dangerous, sending her out there alone, just as it had been dangerous to simply leave Adric where he fell. But then, at least she knew what she was getting into; it had been even more dangerous for the Pharos Project guards, who had no idea who the Master was or what he was capable of.

He felt immensely frustrated at the situation; if it had been simply his own life at stake, he wouldn't have hesitated, would gladly have sacrificed himself to save his young charges.

But he couldn't. Tegan needed him, needed him desperately, and he wasn't looking forward to explaining things to her once she regained consciousness.

Especially if she did so while he was in the middle of draining the blood from her body with his newly-grown fangs.