A/N: I've messed with the timeline a bit – this is set about 2007 – Sierra is aged 27, Rick is 37, and he just met Sophia when he was a bit older, stayed with her a bit longer, and never met Meredith or had Alexis.

CHAPTER 1 – WARNING

A fancy night club, somewhere in New York City...

Rick threw back another whisky, grimacing at the bitter taste as the liquid burned his throat. The night club was loud, very loud, and very dark. He looked over his shoulder at the dance floor; the mass of sweaty, gyrating bodies would have once appealed to him. According to the tabloids, it still did. He rubbed his jaw; it was rough with two days worth of stubble. He turned back to his empty glass and swirled the ice around. The bartender knew him well enough to refill it without being asked; Rick turned in his seat, holding his glass as he once again watched the dancers.

And then he saw her.

Her.

The one.

The one who would change everything.

The crowd parted for her – religious images came to mind, but she was no goddess. Her four inch heels clicked on the floor, her silky black dress flowed over her body, its simplicity highlighting her flawless figure. Her long blonde hair billowed around her shoulders; her black-rimmed eyes pierced his very soul. She walked towards him and he saw her as if in slow motion, noticing every movement, every swish of her hips...

"Richard Castle?" she said into his ear. He stared at her with a gormless expression for at least twenty seconds before he realised she had said his name. She had a British accent. More than perfect... She tapped his cheek.

"Yeah," he breathed. "I'm Richard..."

"Sierra. I'm MI6. Would you please come with me." She might have used the right words, but it was not a question. Her voice... She was used to giving orders. Rick practically fell of his barstool in his haste. She could order him around any time. He followed her out of the bar. The alcohol had made his brain slightly... foggy. But she had said to go with her. She had definitely said that.

There was a black SUV waiting outside. Rick laughed. So predictable – and yet, so cool. She opened the door and motioned for him to get in. He scrambled into the back seat, knowing he looked like a clumsy idiot, but unable to do anything about it. He was hypnotised. He scooted over and she got in beside him; the faceless driver pulled away as she closed the door.

"I don't think your name's really Sierra," he slurred, saying the first thing that came into his head.

"I know your name isn't really Richard Castle. But I'll call you it anyway. Mr Castle, my agency-"

"You're MI6, right? So, a British secret agent - foreign intelligence?"

"Yes. Now if we could-"

"That's just... awesome," he breathed.

She smiled sarcastically. "Thanks. If you interrupt me again, I will hurt you," she added, her tone suddenly icy. He put his fist in his mouth and nodded, shuddering.

"I want to ask you a few questions about a certain CIA agent you've been... tailing recently. Tell me, what is the nature of your relationship?"

Rick blinked. "You know about Sophia?"

"What is the nature of your relationship?"

"She's my... I follow her for research. For my books. I'm a novelist, I write crime novels..."

"I know. I've read them," she said dryly.

"You have?" he asked gleefully.

"We all make mistakes. So, Sophia Turner – is it just research?"

"Well, we-"

"You're sleeping with her. As I suspected."

"No. Well, yes, but wait, how did you-?"

"Mr Castle, I probably know significantly more about your life than you do yourself. I ask simply for confirmation; my sources are not always as reliable as I would like."

Rick shivered. He loved it when she talked. The cool air from the air conditioning was helping to clear his head; he felt vastly more coherent than when he had first got into the vehicle.

"Why are you talking to me about Sophia?" he asked.

Sierra's expression gave nothing away. "As a courtesy. She is about to become the centre of a major investigation by my agency, and it would be beneficial to your... health, if you were to remove yourself from her life."

"What? Why? She's... aren't MI6 and the CIA friendly?"

"We are. Miss Turner, however, seems to be not so friendly as one would hope. Clichéd as it sounds, the less you know the better." She produced a file from under her seat. "This is a document which you will sign, agreeing to never mention this meeting, or anything in it, to anyone. Ever. If you break this agreement, you could endanger many lives – and you will certainly lose your own."

Rick gaped at her.

"Wait, what is it? What's Sophia... why is she under investigation? I'll sign your thing, I won't tell anyone, you can tell me..."

Sierra sighed. She had been here too many times to count. "No." She handed him a pen, and noted in his favour that he actually read the document before signing it. The car stopped moving.

"We're outside your apartment," she told him. "It was nice to meet you."

"Will I see you again?" he asked, desperate to extend their meeting. He had no idea what was going on. He wasn't in love with Sophia exactly, but they were in a very intense relationship, which he still did not have much intention of ending. He was a master of disobeying orders. Even from someone like 'Sierra'. Screw doing what she said – he wouldn't mind if she punished him.

"I hope for your sake that you don't," she said. He kept his eyes on her as he got out of the car, lingering before he closed the door. She gave him a little wave, and he shivered. The way she made him feel... He let himself into his building, gulping for air. Wow. And, what the hell was going on?

…

Sierra leaned back into the soft leather seat and brought a hand to the back of her neck. She tugged off the wig and shook out her short brown hair.

"Where to, Ma'am?" asked her driver, Victor.

"My hotel, thanks," she said, closing her eyes. This job really was full of surprises – never had she expected when she was recruited in 1999 that she would be meeting Richard Castle. Still. She would never see him again... But his face when he'd seen her. He'd almost been drooling. She laughed quietly to herself, the gentle tinkling sound soothing the dull silence of the car. Sure, she had been wearing a wig, but having that effect on a man she so admired couldn't help but give her ego a little boost.