A wise writer once said that beginnings are easy. And perhaps he's correct, if you're telling the stories of ordinary people. This is not a story about ordinary people - and could not happen between ordinary people, because ordinary people don't a) live for billions of years or b) regularly travel through time. This is the story of how Jack Harkness fell in love with an angel named Castiel and how the angel Castiel fell in love with a being known at that time as Captain Jack Harkness. Thus, the beginning is a bit tricky.
So a beginning will have to do and this one is at least chronologically the beginning, and it's also the first time that Jack Harkness meets Castiel. Jack doesn't know this, doesn't know the chap at all and in fact can't see him because it's pitch black inside of the box where his brother has just buried him to leave him to rot for at least the next several hundred - possibly thousand years. "Well, this is a bit of a surprise," he grunts to the man who has materialized on top of him.
"Hello, Jack," says a male voice, raspy but serene.
"I'm afraid you've got the advantage of me," replies Jack, adjusting himself. Jack knew it was going to be some time, and was trying to get comfortable, and while he admits that the decidedly firm masculine body pressed on top of him is a nice surprise at the moment, if it's going to be there for the next couple hundred years, the pins and needles are really going to be a bitch. "Dare I hope you're here to rescue me?"
The man on top of Jack hesitates and shifts. "This is a somewhat compromising position."
He's right. The stranger smells kind of nice and is indeed very firm, so Jack's body has gone and done the predictable thing, and things are firming up all over. "Well, you've got me there, but if you can get us out of here, I can make it a lot more fun." Maybe Jack shouldn't be wasting the oxygen, but more will trickle in, sooner or later.
"I'm afraid I'm not here to rescue you," admits the voice. "There are laws of causality that must be obeyed, and, of course, it is strictly forbidden to meddle with past events for personal reasons."
Jack laughs. "Of course. Please tell me you're not from the Time Agency," he snorts. It's unlikely - the Time Agency never had the technology to teleport with this kind of accuracy and while the idea that Mr. Firm Body and Whiskey Voice is here for personal reasons is intriguing, Jack really can't place either from his vast database of previous personal encounters.
The man nods and Jack can feel stubble brush against his own chin. "No, I am not a Time Agent."
"Fair enough. So, then, other than to cop a quick feel, can I ask what you're doing in this particular coffin? 'Cause it's a bit crowded, I gotta say."
"Mercy." It's a bit of a struggle for the man to pull his arm up, but Jack doesn't much mind. "I'm here to do you a great mercy, Jack Harkness." And Jack is just about to ask what kind of mercy a man in his position can expect when the man touches two fingers to Jack's forehead and commands, "Sleep."
Jack sleeps through the entire confinement, right up until the shovels are tapping the top of the box to exhume him. He never did catch the chap's name, and by the time he's seeing daylight again he's willing to chalk the whole scene up to the oxygen-deprived hallucinations of a mind overloaded by being buried alive and suffocating for centuries. Castiel never admits it happened (he's a stickler for the rules he had bent, and for him it's the middle of the story anyway), and Jack never thinks to ask: so as far as he's concerned, they don't meet until several thousand years later.
Really, as a beginning to their story it's kind of crap, but it's also kind of true, and that's the way these things work out sometimes. The Doctor is fairly certain that it happened - why or how no one else knows, no one else knows about it - but the Doctor is sort of a side player in these proceedings, an amused observer; however, he is enough of an observer to hear the proverbial tree falling in the forest so that we know that this was indeed the beginning.
There's also an end to the story - well, sort of an end, because with Castiel in the picture there is no such thing as a true ending. The end takes place several billion years and a number of light years away from where the story began. At the end, Jack is an enormous disembodied head in a jar and Castiel notes this condition right off the bat.
"And you still look like an out-of-work accountant," says Jack, or the Face of Boe, as he has been known for the last few million years.
Castiel tilts his head. "You've never objected to it."
The Face of Boe sighs, the smoke surrounding him thickening and obscuring his expression for a few moments. The body of James Novak forces Castiel's vision to mist for a second before Castiel makes it stop. There's no point in getting maudlin - though this is an end, Castiel knows as well as anyone that there's no real ending for them, not for a very long time. "Even out-of-work accountants can have a fantastic ass," says the Face of Boe.
Castiel lets himself smile. "Your innuendos were more effective when you could say them in less than three minutes." He looks down. "Also, are disembodied heads supposed to have libidos?"
Their eyes meet and they say at the same time, "Only you." It's a familiar comfort, the recurring refrain of their time together, tossed in jest, in frustration, in contempt, and a hundred other emotions in tens of thousands of conversations over the course of billions of years. Neither of them would be able to say exactly when it became a promise as well.
For several minutes they're silent, Castiel's hand pressed against the glass, the Face of Boe's smoke swirling around.
"You're dying," says Castiel.
"I know. The Doctor will be here soon, but I'm glad that you came first. Will you stay?" The words are torturous to say, but the Face of Boe puts a note of hope into the last question, even though Castiel is shaking his head even as Jack asks.
"No, this Doctor doesn't know me yet, he won't understand." Jack marvels for a second at how casual his angel's manner of speaking has become, so different than the straight-laced sheriff of heaven that he'd met ever so very long ago. "I-" and Castiel's voice cracks before he continues. "I will be there to greet you, though."
If the Face of Boe had a throat to clear, he would. "You swear that it's there, that I'm going there?"
Castiel's eyes are limpid and beautiful when he's annoyed. "I thought you resigned yourself to my existence long ago. Is it so much to ask you to believe in heaven?"
"No," admits Jack. "It's just - I am very tired."
Castiel kneels and rests his head against the glass. "I know."
"Will I be able to rest, there?"
"That is good." The Face of Boe's eyes drift shut. "Will I look like this, or like I used to?"
"That would be telling." Despite the joke, when Jack opens his eyes, Castiel's face is as serious as it's ever been. "You'll be you. And those whom you miss will be there too."
The Face of Boe chances a smile. "It's been a long time. They'll be able to hold out a little while longer. Meet me alone."
To Jack's amusement, Castiel actually blushes and adjusts his tie. "I can do that," he says.
"Good." They can both hear the sound of steps coming up the corridor. "One last chore to complete. I'll see you soon, Castiel."
"Yes," says Castiel, pressing his lips to the glass. "You will."
When Novice Hame and the Doctor turn the corner, Castiel is already gone. A few minutes later, the Face of Boe gives the Doctor one last revelation before he dies. The Earth has predeceased him, and on other planets, Castiel is known by other names, so this is the last time that a being known as Castiel takes action in this universe - unless we're being strictly chronological, but those events actually take place in the middle of this story - it's that kind of story. Nonetheless, in many ways, this is the end.
Of course, Castiel kept his promise, and even as a disembodied head Jack never lost his libido, just the most obvious anatomy required to act on it - it was some time before anyone else noticed they were in heaven and when Dean Winchester did finally figure it out, he was horrified to catch them in flagrante delicto in a position that's not physically possible on this plane and didn't speak to them for a week afterwards - so it's not really an end either. But we'll not concern ourselves with their heavenly activities - the keyboard might spontaneously combust. As such, we'll call this the end of the story.
Going back to the words of the wise writer, the truth is that both beginnings and ends can be tough. But the good stuff, the meat of the story, where people grow and change and sometimes, when they're very lucky, fall in love - the best parts - those parts are always in the middle. The story of how Jack and Cas fell in love over the course of about ten million years, well, it has a lot of middle, so it's sort of inevitable that the real start to this story is in media res.