They agreed to take the TARDIS to the source of the tyron energy. Luckily no one was in Castle's flat when they got there, so they were able to watch as the Doctor slipped into the invisible TARDIS without any awkward questions from Castle's family.

A few moments after the Doctor had closed the door behind him, the blue police box materialized into view once more. Castle grinned at the sight of it, as did the Doctor once he came back out to join them, but Esposito and Ryan looked less than impressed. "That's your spaceship?" Ryan asked doubtfully.

"You couldn't fit three of us in there," Esposito objected.

The Doctor just smirked. With a snap of his fingers, the doors swung open, and he gestured toward them with a flourish. "Go and see for yourself."

Exchanging a baffled look, the partners approached the box. Castle and Beckett followed, both wearing a smug look. Now it was someone else's turn to be wowed by the TARDIS.

Their reaction when they walked in was priceless. Esposito stopped dead, and Ryan let out a low "Whoa," under his breath. Both looked around the console with wide eyes, trying to take in the amazing sight. Ryan's mouth opened and closed a few times before he finally managed a weak, "Well... I wasn't expecting that."

"It's huge!" Esposito breathed, blinking wildly, as though convinced the TARDIS would grow smaller again in the blink of an eye.

Castle came up behind the partners, slinging arm arm over each of their shoulders. "Gentlemen," he said with a grin, "welcome on the TARDIS." He held back a chuckle at their awed expressions.

"Right." They turned to see the Doctor hurrying through the door and to the console. He dashed around the console, throwing up switches and pulling levers, the familiar manic grin on his face. "I'll land us a few hundred feet from the source of the Tyron energy. You four can pop out and find a way to follow us while Beckett and I take the TARDIS straight in."

Memories of his last flight in the time machine came to mind, and Castle turned to the detectives. "Word of warning, it's a bit of a bumpy ride."

"That's because he doesn't drive her properly," River spoke up from by the door. "She has stabilizers for a reason, sweetie."

The Doctor threw a quick scowl in her direction. "They're boringers!" He complained. "They're just big blue boringers." River just sighed affectionately in response.

Castle left the partners behind and crossed over to where Beckett stood leaning against the railing. "I wish I was going in there with you," he said quietly. He hated seeing her go off into danger without him, even though she was the more experienced of the two. He also wished that he'd had have his partner by his side while they wandered into the unknown, shadowy den, but she needed to be at the Doctor's side, and he had to be her back-up.

Beckett seemed to understand. She gave him a quick, warm smile. "I know. But we'll be fine. And I need you to keep an eye on the boys."

Castle just nodded. He didn't voice his other fear; that the Doctor might make a better partner than him. It wasn't even a real fear. The writer had grown beyond actually worrying about being replaced. He knew that he and Beckett were partners and that was that, but he couldn't help that little irrational worry that somehow the Doctor, with his time travel and his crazy stories and his journeys to the stars, might take his place.

There must have been some hint of it in his face, for Beckett's expression softened into a warm smile. Castle went to pull her into a hug, but Beckett surprised him by pulling him into a long, proper kiss, regardless of Esposito, River, and Ryan's smirks and the Doctor's mix of amusement and disgust.

When they pulled back, Beckett was grinning at him. "I'll come back," she promised simply. And Castle believed her.

This time Castle was prepared for the massive shudder that ran through the ship. He and Beckett were already holding on to the railing, but to his amusement Ryan and Esposito were thrown off their feet. The ship jostled around for a while before jerking to a halt, again lacking the vworpp, vworpp that it had given the first time they'd seen it.

The Doctor threw down a final switch. "Right then, here we are, right outside the lion's den." He gave his monitor a quick look. "Hmm, 'den' looks about right. It's some sort of cave."

The humans crowded around to get a look at the monitor. It looked like a very foresty area with a large cliff face. A wide, gaping cave mouth was cut into the rock. Castle's eyebrows flew up at the sight. "Well that's probably the most stereotypical evil lair I've ever seen."

The Doctor hurried to the door. "Alright, everybody out, let's get going."

Esposito, Ryan, and River headed out the door, but Beckett held her partner back for a moment. "Here," she said as she pushed her gun into his hands. "I get the feeling the Doctor won't want me to have it anyway. Be careful." Castle nodded and gave Beckett one last smile before following them.

She'd be fine. He'd make sure of it.

SCENEBREAK

Beckett watched as her partner and the others exited the TARDIS. Now that she was alone with the Time Lord and his machine, she felt a sliver of self-doubt. They had no idea what they were about to walk into. It could be anything. What if they already knew who the Doctor's companions were? What if they were found out?

The Doctor's voice broke the detective out of her thoughts. "So, we've got an unknown beastie trying to lure me into a secret cave with unknown intentions?" His grin widened as he began to work on the console again. "Let's go and poke it with a stick."

Beckett forced a grin at this display of enthusiasm, so like Castle. But he wasn't her partner, and Beckett couldn't help wishing that the writer was going into this with her. They weren't just dating, they were partners. They watched each others' backs and trusted each other implicitly. She wished that she was walking into this unknown danger with the man she trusted with her life, but instead she got the Doctor.

But Castle would still be there, waiting in the shadows to be her back-up, and she knew that as long as they were both there they'd both be safe. Because there was no way she was letting anything happen to him either.

The Doctor seemed to notice her silence. "Are you alright?" He asked with concern. There was nothing condescending in his tone, just genuine concern.

Beckett straightened, looking the Doctor dead in the eyes. "I'm fine," she told him confidently. "Let's go get this son of a bitch." The concerned girlfriend was gone, leaving the cool, savvy detective in her place.

The Doctor's grin returned full force. "Extrordinary Kate Beckett. The aliens don't know what's coming for them." He raced over to the console to throw up one final switch, sending the TARDIS back into violent shudders. "Geronimo!"

SCENEBREAK

When they stepped out of the TARDIS, they found that they were fairly deep inside the cave. A hole in the roof far over their heads had sent sunlight spilling into the cave like a spotlight, with the edges of the cave still consumed in shadows. There was no alien in sight, but as they closed the door of the blue box behind them, a sharp hiss sounded out of the shadows. "Welcome, Doctor, and your human companion. I've been expecting you two."

Beckett tensed in anticipation, but the Doctor just gave an easy grin, bouncing forward on the balls of his feet. "Well hello there. You must be friends with Crah."

"The Fatorin was merely a puppet, Time Lord. It is you I have waited to meet."

The Doctor stared cooly into the shadows, a slight smirk on his face. "So you manipulated a Fatorin into killing his enemy, an innocent Arondrian, then somehow tricked him into thinking it was his idea before killing him? And all to get my attention?" He paused, waiting for a confirmation that didn't come, before saying, "It's a brilliant set-up, I'll give you that. So can we maybe meet the person pulling the strings?" A long hiss followed his words, spewing ominously out of the darkness.

A sleek black form slithered out of the shadows. It was like an enourmous snake, with smooth black skin like blubber, and a rounded head with a tapering snout. Cold ice-blue eyes blined out at him from that face, with long, thick whiskers sprouting from his jaws. The tip of his tail had similar, even thicker whiskers, more like feelers or antenae, which waved lazily in the air. His body was thick as a person is wide, and from his snout to his tail he could have been as long as three busses.

At the moment, his neck was raised so his head was lifted high over their heads, swaying slightly as his gaze fixed on the Doctor. When he spoke, it was in a voice like steam escaping, a long, drawn-out hiss. "Welcome, Time Lord."

The Doctor gaped up at the creature. "You're a wyvern," he breathed. "But that's impossible.

The creature didn't seem to need to blink. Those eyes stayed fixed on the Time Lord with eiirie intensity. "No more impossible than you, Doctor. The last of the Time Lords." Those icy orbs narrowed slightly. "Yes, the very last of your kind. A common title these days so I'm told, and one that I share. I am Torkk, last of the Wyverns." Malice flooded into his expression as his voice dropped to a low rumble. "Thanks to you.

Beckett threw the Doctor a worried glance. "What does he mean, Doctor?"

The Doctor didn't take his eyes off the wyvern as he explained, "The wyverns lived on a planet in the same system as mine, as Gallifrey. They were a hunter-tribe race, didn't have advanced much in technology, but inherently telepathic. They had the ability to influence the minds of others through touch, like Time Lords, but so much more advanced." His eyes shone with a balance of admiration and distaste. "One touch and they could slip right into your mind and leave you thinking you're a pink flamingo with an umbrella for the rest of your life." His brow furrowed slightly. "Though you must be out of practice. Crah didn't take the false memories too well, fried his brain clean out."

Torkk let out a vicious hiss at the Time Lord's insinuations. "We did not use our powers for ill! it screeched, thrashing his tail furiously. "We were not conquerers or manipulators. We used our skills to hunt enough prey to feed our many mouths. We were a large people, and our prey was vicious as we. It is no different than the snake paralyzing its victim with venom. We used our powers to survive!"

Sadness flashed in the Time Lord's eyes as he countinued his narrative. "My people always knew about the wyverns, but we never interfered until the Time War..." he trailed off, his expression becoming one of infinite sadness. Beckett had not believed until now that he was over a milenium old, but now she could see every year on his shoulders like a weight.

Torkk let out a menacing hiss. "What? Will you not tell her, Time Lord?" He lowered his head and neck to the ground and began circling slowly around the four. "Tell your precious companion how your people enslaved mine. How they created great machines to enslave us, to cut us off from our telepathy, leaving us helpess. How your people stole our minds and left us dumb as beasts, then set us loose against your enemies like common creatures of war." Something shook in his voice, like the sound of a rattlesnake. "My people were a peaceful race, and the Time Lords enslaved us!" He circled more furiously around the two, no longer speaking, just letting out a long, furious hiss.

The Doctor watched the wyvern with horror and pity. "I'm sorry," he said softly, "I am so, so sorry."

The wyvern stiffened, pausing in his manic circling, then turned a hateful, blazing glare on the Time Lord. "And you, Doctor. The worst of them all. We thought you pitied us, Doctor. Those of us too young to use for war, those with our minds left. We saw you and thought you were our savior. You were different from the others. We thought you cared."

He drew his head closer to the Doctor, ice-blue eyes still unblinking. "Then you murdered us! You destroyed the wyverns! Gallifrey burned, and we burned with it! You came in with your morals and your rules and you burned your own planet to the ground. You didn't save anyone! My people are dead because of you!" He roared out the last words in the Doctor's face, his whiskers waving uncontrollably.

Beckett froze in shock. He had killed the Time Lords? His own species? She remembered how completely broken the alien had seemed that night he had told her that his planet had burned, and she had felt a connection. She'd felt sorry for him. She'd thought they were the same, grieving over something that was taken from them. But he had been the one to pull the trigger. She turned to him, looking the broken Time Lord right in the eyes. "Is it true?" She asked in a low, dangerous voice.

The Doctor flinched. "I had to," he whispered hoarsely. Beckett felt like she'd been punched. She'd felt sorry for him, she'd thought they were both victims. But he was the murderer. "The Daleks were too powerful, and my people..." his voice went dry, and he had to swallow before continuing. "My people had grown destructive to an unstoppable level. They were going to destroy the entire universe and everything in them. They couldn't be reasoned with. I had to stop them. It was my planet or the entire universe." He looked the wyvern in the eyes as he whispered, "I am truly sorry."

Torkk's hissing subsided, his eyes boring into the Doctor's. "I know. I know you feel guilt, believe me. I know you repent every day for the havoc you have wreaked. And believe me when I say," here he leaned in even closer, "that it is not nearly enough."

The Doctor looked with infinite sadness at the wyverin. "You lost your entire species, and you rage at me for harming innocents," he said quietly, "yet you murder an innocent and fry a Fatorin's brain just to get my attention."

The hissing reached a higher pitch. "I did what was necessary to draw you out of hiding," Torkk spat viciously. "I used a Dalek metal transport beast to escape before the Time War ended. I wandered a long, long time trying to find you, Doctor. When I landed on Akali, they told me that the last Arondrians had fled to Earth. That's when I knew I had you. I stole a Fatorin and forced him to point me to Earth. I knew that the harming of alien innocents on Earth would be the perfect bait for you."

The Doctor shook his head sadly. "Murder is never necessary," he said gently. "You should know that better than anyone. But here you are, the righteous defender, with innocent blood on your hands. Friedrich Nietzsche saw that once. 'He who fights with monsters might take care lest he thereby become a monster.'" He let out a sigh. "Look what you've become, Torkk."

There was no regret in the wyverin's eyes as he hissed, "Do you think I care? All I want is to see you suffer, Doctor. And I plan to."

His tail tip snaked out and touched the Doctor's forehead, resting there. The Doctor stiffened, his eyes widening. Torkk let out a fierce hiss. "I can see your mind, Doctor! Oh, a difficult mind to be sure, harder to break. The great Time Lord mind, enough to hold galaxies. And as you say, I am out of practice. This will take time. But no matter, no matter. For I have seen your soul, Doctor, and I know just how to break it. Shall I show you?"

The Doctor's body went completely rigid, his mouth opening in a silent scream of agony. Torkk let out a delighted hiss. "Yes, Doctor! Feel it! Every life you've ever stolen. Every dark act you've committed. Every lie." The creature leaned in closer, hungrily taking in the sight of the Doctor's agony. "I can see them all, Doctor, can you? The Time Lords, the Racnoss, the autons. Daleks, cybermen, silence, sontarons, the list has no end. What a legacy you leave, Doctor."

The Doctor crumpled, falling onto his knees, his hands covering his face. The wyvern let out a cruel screech of laughter. "'The Doctor.' The man who fixes people. The wise man, the healer. Even your name is a lie. Shall I tell you your true name? The Oncoming Storm. The Destroyer of Worlds. The Predator. You are the man who destroys everyone, even his friends." The creature's gaze shifted briefly to the horror-struck detective. "You surround yourself with humans, Doctor, and you cut even their tiny lives short. Every human who has followed you has been lost. Sarah Jane, Martha, poor lost Donna. Even the pretty blonde, the girl who foolishly forgave you for the Time War."

The Doctor let out an agonized moan. "Rose..."

Torkk shrieked gleefully. "You lead your companions like lambs to the slaughter, and you do it again and again! Feel it, Doctor! Their fear! Their pain! Their blame! For you, Time Lord, are the reason for their suffering. Feel it all, Doctor, and remember the wyverns!"

Beckett watched the Doctor's agony uncertainly. A small part of her brain wondered if maybe this was the Doctor deserved. The murderer of an entire planet and more than one race. The choice had faced had been a hard one, yes, but as hard as she tried, she couldn't imagine being the one to pull the trigger on her mother, no matter the price. It was too deep a pain. How could the Doctor live with what he'd done?

But at the same time she knew that the Doctor really was trying to help, and no matter what he'd done, she couldn't let him die. It was her job as a cop to protect him. But without a gun, there was nothing she could do. She was too small compared to him, and she couldn't ask the Doctor what his vulnerabilities might be.

Where are you Castle? she wondered desperately. I could use one of your theories right now.

SCENEBREAK

Castle had followed River and the detectives into the darkness of the cave. They had filed silently through the darkness, each wondering what they were going to face, until they'd reached a wide cavern. There were some higher ledges where they could oversee what was going on, so they clambered up as silently as they could and waited for the TARDIS to arrive.

As the TARDIS arrived and the scene unfolded before them, Castle watched the wyverin with interest. This wasn't a conquerer here to invade. This was a victim fueled by anger and revenge. Somehow, that made him seem infinitely more dangerous. A cornered animal was more unpredictable than a calculating hunter.

The creature's accusations shocked Castle as well as his partner, but the Doctor's explanation held a ring of truth. His own people or the universe. What a terrible choice to face. He could understand why the Time Lord had made his choice, but he knew Beckett might not. After the way she lost her mother, she might not be able to accept what the Doctor had been forced to choose, and Castle got that.

But he also knew that she would do whatever it took to stop Torkk from killing the Doctor.

Esposito started to raise his gun, but River reached out and forced it down, shaking her head. "Kill him or wound him and the Doctor stays like this," she hissed in a low voice. Even in the darkness, Castle could hear her fear for her husband in her tone.

As he tried to think of a way to help, something from earlier in the conversation came to him. "They had the ability to influence the minds of others through touch, like Time Lords, but so much more advanced.." He'd also said that the wyverin was out of practice. Maybe having too many memories forced on him would disorient him?

Castle reached out and put a hand on both detective's shoulders. "Do you trust me?" He asked in a low whisper.

Esposito and Ryan nodded immediately. River thought for a moments before giving a firm moment. Castle explained, "Then do what I do. When you do, remember as much as you possibly can. Remember everything."

Before they could ask him to explain that baffling statement, Castle was scrambling down from the ledge's and racing towards the wyverin. He could hear Beckett calling his name briefly before he reached out to touch the wyverin's vast side. Then his world exploded into pain.

Memories that were not his we're being forced into his head - memories of pain and vengeance from the wyverin, and memories of grief and guilt from the Doctor. The pain and death of millions. With a fierce effort he ignored them and called his own memories to the front of his mind. Growing up with Martha, becoming famous, Meredith, Gina, the birth of Alexis, raising his daughter, the first time he met Beckett. All the pain and sympathy he'd felt for all the murder victims, all the families. The love he felt for his family, and for Beckett. He pulled up all of his strongest thoughts and emotions and pushed them at the wyverin.

"What are you doing?!" the wyverin hissed furiously. Castle didn't answer, just focused on his memories. He heard footsteps as the rest of the team raced over, and he felt the surge of new memories as the three detectives and River touched the wyverin.

Case after case from Ryan and Esposito, the grim sadness of working with death. From River he felt the terror and training of her early life, then the confusion and love of meeting the Doctor, the dizzying fear of one day losing him. The Doctor's guilt was easiest thing to feel. The guilt of a million deaths, all huddled on his shoulders, and he thought he deserved it all. That pain screamed out through their telepathic connection, but Castle ignored it and focused on Beckett. He could feel the agony of losing her mother, the mask of being okay, the way her life slowly pieced together again. And he felt her love. Her love for him sang through his mind, and he focused on that, tried to block out everything else, to keep his mind from going insane with all the voices in it.

Suddenly, the Doctor grew stronger in all their minds, then the connection was severed. As the fog of memories cleared from Castle's vision he could see Torkk slither back as though he had been struck. His ice-blue eyes were wide and crazed, his tail thrashing wildly. "What have you done?!"

"I changed you." The grim answer came from the Doctor. The Time Lord still looked deathly pale, and he had to lean against River to stand, but his expression was one of grim judgement. "Your abilities aren't as strong as they once were. While they distracted and overwhelmed you, I went into your mind and locked your telepathic abilities inside your mind. You won't be able to reach inside anyone else's mind ever again."

The wyverin thrashed his head furiously from side to side, an ear-piercing shriek issuing from him. "You took my protection!"

The Doctor shook his head. "I took your weapon," he amended gently. He took a step towards the wyverin, his gaze imploring the alien to see reason. "Now let me help you, Torkk. There's been enough loss already. You're right, the death of the wyverins is my fault, but I'm not about to finish that genocide. I can find you somewhere safe to live. Please, Torkk. Let me help you."

The wyverin shook his head violently, his glare fixing on the Doctor once more. "There is nothing left for me, Doctor. All I have left is destroying you." He lunged at the Time Lord with teeth bared.

Four gunshots rang out. Castle couldn't have said which one of them had hit their target, but the creature jerked back with a sudden scream, then collapsed against the ground, his thrashing growing steadily weaker.

The Doctor looked stricken, but he didn't turn to yell at anyone about the use of guns. Instead he knelt by the wyverin's head, eyes heavy with sorrow. "I'm sorry," he said quietly. "I am so sorry."

Torkk blinked back at the Time Lord, hatred burning in those icy blue orbs. "Remember this, Doctor. You are responsible," he rasped, blood gurgling in the back of his throat. "Another genocide to add to your glowing legacy. You murdered me, Doctor. You killed the last of the wyverins. My blood... is on your... hands..."

The creature's eyes rolled to the back of his head, and his head fell to the ground with a dull thump. He let out a few last, shaky breaths before stilling, his eyes becoming glassy and dull. The last of the wyverins was dead.

No one spoke for a long time. Without a word River came to stand beside the Doctor, wrapping her arms around him from behind in a silent hug. Castle and Beckett exchanged a solemn glance before joining River by the Doctor's side, soon joined by Esposito and Ryan. Silently, the small team stood huddled together, joined in the memories they had just shared, yet each wrapped in individual grief. They stood in mournful silence over the last of the wyverins, and wondered at what their victory had cost.

So here we are at the end. There's going to be one last chapter to wrap up loose ends, but the mystery is solved, and the culprit dead. I hope you've liked how the mystery turned out and how everything was wrapped up.