Beckett still seemed a little on edge when they reached Castle's flat. The detective had a set to her jaw that showed her disappointment. The Doctor seemed to notice, because after they all exchanged weary greetings, he put a hand on both her shoulders and looked her in the eyes. "We'll find them, Detective Beckett. That's a promise."

The detective stiffened at the sudden contact, eyes wide. After a few moments she relaxed slightly before pulling away. "No need to patronize me, Doctor," she said wearily, no real bitterness behind her words. "We'll pick it up tomorrow."

"Oh, hello again," a familiar voice came from upstairs. Martha Rodgers descended from the stairs, this time with a ginger teenage girl in tow. "I didn't realize you two would be staying another night." She swept almost regally into the room, a permanently dramatic air about her. She wasn't like anyone River had met before, but she found herself liking the actress.

The teenager looked curiously at the time travelers, displaying none of the hostility of Tanya. "Who's this?" She asked Castle.

The man wrapped an arm around the girl's shoulder in a completely familiar matter. "Alexis, this is Doctor River Song and her friend the Doctor. We brought them in to help with our case, but they didn't have anywhere to stay. Doctor, River, this is my daughter Alexis." For a moment, his arm tightened protectively around the girl, and his eyes gazing at the time travelers suddenly gave a flicker of distrust. Of course, River realized, we've proved ourselves enough for him to trust us for his own sake, but his daughter's another story.

River forced herself to give a warm smile, holding out a hand for Alexis to shake. "Good to meet you, Alexis," she greeted the girl, shooting Castle a quick, reassuring glance. "Your dad was very generous to let us stay the night."

Alexis shook the offered hand with a smile, but she seemed to sense something was being held back. "So, Doctor Song and Doctor…?"

"Just 'the Doctor.'" The man in question answered, staring at Alexis curiously. "You weren't here last night," he remarked in a way that made it seem like a question.

Alexis shrugged, still eyeing the Doctor warily. "I'm just visiting from college," she explained. "So you actually just call yourself 'the Doctor?'"

The Time Lord gave an easy nod. "Yes, just the Doctor. Why is everyone surprised that it's just the Doctor?"

Alexis didn't answer, instead, giving her father a questioning look. She didn't seem to distrust the time travelers, but she did seem to realize the whole truth wasn't being shared. "So what's the case this time?" she asked instead. "And how are you two helping? Is there something off about the body? The NYPD doesn't usually get outside help, except for Dad, so you two must be higher ups."

Ooo, River liked this kid. She was curious, smart, and asking the right questions. She had a feeling Alexis would make a good companion, but she also had the feeling Castle and Beckett would take the Doctor's head off his shoulders if he ever tried to recruit her. She shook her head with a slight smirk. "Sorry kid, we're not that kind of doctors. I'm an archaeologist. We've been traveling a bit, seen a lot, and we've got a lot of experience with this kind of case." The Doctor let out a slight chuckle, and Castle and Beckett seemed to be holding back smirks at River's word choice.

Alexis, however, still looked curious, but she seemed to recognize that River wasn't going to say any more on the matter. "Well, nice to meet you," she said with a smile, which River returned. Alexis shot her dad a quick glare which seemed to say I'll be asking you about this later. Oh, she definitely liked this kid.

SCENEBREAK

That night, the stories at dinner had to be reigned back a bit. Now that Martha and Alexis were present, River and the Doctor couldn't exactly bring up aliens and time travel. However, River seemed pretty adept at exchanging planets and species inside a story for ones more plausible for Earth. The stories were undoubtedly still the fantastic tales of their journeys through time and space, they were simply a little more Earth-themed now.

In turn, Beckett and Castle shared some of their more interesting exploits. Beckett was surprised at how interested the time travelers seemed, especially the Doctor. She'd thought after all they'd seen and done, some homicide cases would be a bore to them, but they seemed truly interested in the cases she and Castle related to them.

After a particularly interesting story about a man found hanging under a bridge, Martha turned to a still-laughing River and Doctor. "Well, you two certainly seem to lead an interesting life. How did you two first meet?"

To Beckett's surprise, River looked suddenly uncertain and sad, while the Doctor stiffened, fierce pain flashing in his eyes for a brief moment. He shook his head slightly, the old smile returning to his features, but somehow it seemed empty. "Spoilers," was all he would say.

River looked at the Time Lord with something like fear in her eyes before turning back to a bemused Martha. "I was pretty young when I met him," she said with a slight smirk, as though laughing at a private joke. "I was a pretty rebellious teenager. Oh god, I was always getting in trouble. Then he came along and got me interested in archaeology."

Martha raised an eyebrow. "Oh? How so?" Beckett held back a smirk. Castle had certainly gotten his talent for sensing good stories from his mother.

River and the Doctor shared an amused glance. "Well, it's a bit of a long story," the archaeologist said, choosing each word carefully. "You see, I may or may not have tried to kill him the first time I met him."

Beckett stiffened in shock. She was sure her face matched the dumbfounded expressions Castle, Alexis, and Martha wore. The cop in her immediately recoiled in distrust, but she forced the instinct back. She'd known River and the Doctor had a colorful past, after all. She just hadn't been expecting that. A quick memory of "Detective Cage" pointing a gun at them with a feral smile crossed her mind, but she forced it back.

Alexis was the first to speak. "You… tried to kill him?" she asked hesitantly. The unspoken question hung in the air. Then why are you sitting together now?

The Doctor grinned cheerfully. "Yes, well, that's the thing about me, no matter how many times you try to kill me, I always seem to spring back." Affection, and something deeper, gleamed in his eyes as he turned to River. "She tries to kill me, then saves my life the same day. Bit of mixed signals there. After that, you just couldn't resist, could you?" For a moment, Beckett was surprised at how openly the Doctor was flirting with his companion, but then she realized River's smile had faded slightly and her hands were gripping the sides of her chair tightly.

The Doctor's words finally brought River's smirk back. "I'm the one who couldn't resist?" She asked with amusement. "I'm not the one who broke into a military base to save you."

The Time Lord pouted slightly. "I didn't know you were there," he protested, "I was trying to save Amy."

This time it was Castle who asked the question. "Amy? Who's Amy?"

The Doctor's eyes glazed over with sadness, but River seemed more open to talking about it. "Amy and Rory Pond. They're… friends of ours. We all travel together. Well, they travel with him; I just hitch along whenever something fun's going on. Or when the Doctor's gotten himself into something he needs me to get him out of."

The Time Lord tried to scowl, but an affectionate smile ruined it as he protested, "You're the one who's always calling me to help you out."

Alexis looked between the two, understanding flashing in her eyes. "Where are they now?" She asked gently. "Amy and Rory I mean."

The Doctor gave a smile that didn't quite seem to reach his eyes. "They stayed behind. They're safe at home."

After a few awkward moments of sympathetic silence, the topic was hastily changed, but Beckett couldn't help but wonder what the Doctor meant by "left behind." From what she'd gathered, Amy and Rory were human. What happened to the humans who traveled in the TARDIS? The Doctor had dropped so many names – Ace, Jamie, Martha, Donna, Rose, and now Amy and Rory – but never explained why there seemed to be a different companion for every story. His life seemed to be an exceedingly dangerous one.

Is that what the Doctor did? Pick up someone to ease the loneliness until some danger took them away as well? What was it like to come to care for someone that much and then to lose them over and over again? Subconsciously she reached out for Castle's hand, only relaxing when he took the offered hand. I'm not losing you like that, Beckett thought fiercely. Not ever.

SCENEBREAK

After dinner, Alexis and Martha excused themselves for the night. The Doctor had headed into the still-invisible TARDIS to tinker with something, so Beckett and Castle took the opportunity to question River. "What was the Doctor talking about with 'spoilers' earlier?" Castle asked curiously.

River let out a sigh. They were all seated on the couch in the living room, with the partners both turned to face the archaeologist. She looked up at the partners with a sad smile. "I told you earlier that I don't travel with the Doctor permanently, I just come along sometimes." At Beckett and Castle's nod she continued, "Well, he's a time traveler. We don't always meet at the right time on each other's timelines. Sometimes he's come to pick me up and it'll be the same Doctor I had an adventure with a few days ago, and sometimes it's a Doctor who's only met me a few times and doesn't really know who I am." The woman leaned back against the couch with an unusual air of sadness. "There are adventures we've shared that he hasn't lived through yet. So many memories I can't share with him because they're still in his future. And the same goes for me. I've never lived through the first day he met me.

Beckett's eyebrows went up in surprise. And she'd thought her relationship with Castle was complicated. "So sometimes you have to meet a version of the Doctor who doesn't know you?"

River just gave the same sad smile. "Life with a time traveler," she said quietly. "His past in my future. Every time I meet him he knows me less."

Castle cast a glance at the place where the invisible TARDIS rested. "What about this version? He seems to know you pretty well."

River's expression softened, and warmth lit up her eyes. "Yes. He knows me pretty well right now. He knows who I really am, and he knows my secrets. But there's still so much we haven't lived through together. And one day I'll lose him forever." That fear from earlier filled her eyes again. "One day, I'll have to live through that first meeting. The day the Doctor looks at me and doesn't have the faintest idea who I am. He'll never have met me at all, and I'll be stuck with all the memories I can't share, knowing that I'll probably never see him again."

Beckett found there was nothing she could say to that. What could possibly be said to make something like that better? To live with that kind of countdown, knowing the end was coming and there was nothing you could do to stop it…

Unconsciously, Beckett shuffled a little closer to Castle, leaning back against his shoulder. They may have wasted four years, but at least they had all their futures stretching out ahead of them, to use as they wished. River didn't even have that.

A few moments of silence passed before River spoke up again. "Alright, as long as we're having this little heart-to-heart, what about you two? You're obviously together, but what's the story there?" She cast a pointed look at the way Beckett was almost snuggled up against Castle.

She shuffled away a little self-consciously, but there was no reason to hide anything from River. "Yeah," she said simply.

Castle gave a warm smile. "We've been together a few months."

Beckett looked down at her hands for a second, avoiding River's gaze. Their relationship wasn't something they really talked to other people about. "Coworkers aren't really supposed to date," she explained a little awkwardly. "We've been keeping it a secret."

River nodded in understanding. "Ah, forbidden romance. Fun stuff." Her gaze seemed far too knowing for Beckett's liking.

Eager to move the conversation away from them, Beckett asked, "So what's it like traveling with the Doctor? Going anywhere and anywhen?"

River leaned back against the couch with a wistful, slightly awed smile. "Traveling with that man is like flying. I wish there was a better way to say it. You can go anywhere, meet anyone, touch any star, tied to nothing and no one. You can save galaxies, become a hero, actually change lives. And all in the company of that mad, brilliant man and his box."

She gave Beckett a quick, knowing look. "I know you've been wondering about all his companions. He's dropped a lot of names, more than he usually does. He's lived over a thousand years and picked up so many friends to travel with, and lost them all. Life with the Doctor is extremely dangerous. I won't lie and say it's not. Some companions die, some are lost, and some leave for their own safety. I can get wondering why anyone would want to choose that life." She smiled as she cast a loving glance at where the invisible TARDIS sat. "But to see all those stars, all those skies, and to run with that man, is one life really such a price to pay?"

River's words painted wonderful pictures of far-off worlds and long-lost sights. Beckett was spellbound in a way she seldom was. The life River was describing seemed almost too good to be true. To be able to go anywhere, help anyone who needed it, save as many lives as time could hold…

Wasn't that, deep down, what she really wanted?

SCENEBREAK

The next morning woke Castle later than he usually woke. He lifted a groggy head, wincing as the early morning light hit his eyes. Vaguely he wondered why he felt so tired, before remembering. River's words last night had inspired him so thoroughly that he'd had to write. He'd stayed up a better part of the night furiously typing all the fantastic words and ideas that swam in his head. Distant worlds, fantastic creatures, the company of a god who acted like a child. Of course, he didn't reference any of River or the Doctor's stories directly. He wasn't even sure he was going to use any of it for any book. He'd just needed to get it all down and let his imagination run away for a bit. And now it was all down if he ever felt inspired to finish it.

At the heart of it all was the tragic story of River and the Doctor. Two people tied together by love, but never truly together, losing each other day by day. A true tragedy. Castle had explored the consequences of such a story through his writing last night, and no ending he could see could lead to either being happy.

Internally cursing himself for staying up so late, Castle forced himself out of bed and onto his feet. When he went out into the kitchen, all he found was a note saying Beckett, River, and the Doctor had already left for the precinct.

"Hey Dad." Castle turned to see Alexis coming down the stairs behind him.

"Hey," Castle greeted warmly, wrapping his daughter up in a quick hug. "What are you doing up so early?"

"I wanted to catch you before you left." The redhead hesitated for a moment. "Dad, is there a reason you don't want to talk about this case?"

Castle stiffened, then turned to the fridge hurriedly, not looking at his daughter. "Who says that I don't want to talk about it?"

Alexis rolled her eyes. "Dad, please. You wouldn't answer any of our questions about it last night. You just passed them off or ignored them. You always tell us about your cases. So what's going on? The doctors didn't look like government officials, but they wouldn't say anything either."

It was times like these that Castle wished his daughter were just slightly less brilliant. He loved her, but it was hell trying to lie to her. He tried to make his voice casual as he told her, "It's nothing, really. Some of the circumstances were a little weird and Gates wants us to keep it close to the chest. And I really don't want to piss her off more than I already have to." He mentally thanked Gates for providing a good excuse for his silence.

Alexis's brow crinkled. "Weird how?"

Castle scanned his brain desperately for something. "Like it was professional. The vic might've been mixed up in some really tangled political thing. It's complicated, and I'm sorry, but I really can't tell you."

Alexis looked at him searchingly for a few more moments before sighing. "Alright. Just stay safe, alright?"

Castle smiled. "I promise."

As Alexis turned to the kitchen to get her food, Castle thought about what River had said the night before. "Life with the Doctor is extremely dangerous. I won't lie to you and say it isn't." The case they'd gotten into was wild and amazing and fantastic, but it was also unbelievably dangerous. They could be possibly facing an entire alien invasion, and it was just the four of them – seven now that Lanie, Esposito, and Ryan were in on it – against these terrible odds. As thrilling as that was, it was also probably the most dangerous case he'd ever been involved in. He'd choose to help solve this case no matter the danger, but there was no way he was letting his daughter get swept up in all this. He couldn't tell her the most amazing thing he'd ever learned.

He only hoped his silence wouldn't put her in more danger.

Have some Castle, Beckett, and River POVs all thrown in together. As for the Doctor's POV, that's a challenge I'm not sure I'm ready to tackle, so don't assume it's going to show up at any point in this story.

If River seems at all off, I'm really sorry. I haven't seen an episode with River in a while, and it's hard to write her in some of these more domestic situations. She's usually there right in the thick of the action, not sitting off to the side with the victims.

More soon, and there'll be more Esposito, Ryan, and Lanie.