Set in 7x06 "Time of Our Lives" parallel universe where Castle and Beckett had never met before, Beckett is captain, etc. Castle remembers an important date.
Castle had weaseled his way into working with Beckett at the precinct the same way he had done in their original timeline, by using the pretext of research as his cover and basing a new series of novels on her. Since Kate Beckett was captain of the 12th precinct in this universe, he had to use his friendship with the mayor to go one step above her head, the commissioner, to get official blessing to consult on cases.
Everything was slowly falling into place as it should be with them, and Castle got the impression that this Beckett was taking a liking to him much faster than she had in their original universe. Of course, there was that façade of annoyance she put up for show, but he could tell she genuinely enjoyed his presence at the precinct, even at times when he simply kept her company while she did paperwork. This Beckett was quite receptive to him and much more tolerant of his antics, given that she hadn't thrown him out of the precinct yet despite having the authority to do so.
And if he wasn't mistaken, those glances she cast at him when she thought he wasn't paying attention definitely had a hint of desire that he hadn't noticed, or simply missed, in the early days of when they met the first time around. She quickly turned away anytime he caught her, but the small smile lingering on her lips was a tell that gave her away.
Although he was sad not to already be with Beckett romantically in this new timeline, the prospect of falling in love with her all over again was one Castle was looking forward to. That was the only bright side of being stuck in this alternate universe where everything in his life was turned on its head.
Beckett would never admit as much, but she was developing a soft spot for her civilian consultant. He fit into the precinct culture so well and got along with the boys famously, as if they had been longtime friends. Just six months after Castle had spontaneously arrived, he had quickly become part of their little precinct family and she was already feeling like he was one of their own.
The fact that she was warming up to him was probably just due to him bringing her coffee with a smile every morning, or so she told herself. But if she was being honest, those feelings might run deeper than merely a professional admiration and respect.
He was a gifted storyteller who spun all sorts of wild theories that made their cases a little more fun. Of course there was the craziest theory of all, that he was a time traveler from an alternate universe where they had been partners, and more than that allegedly, for years. She would have dismissed this ludicrous tale entirely as just one of his fantasies, but there was something particular about him she couldn't quite place which kept nagging at her.
He could have just been some obsessed stalker who did his homework on her, but there were things that Castle knew that he couldn't possibly have learned from the news or the internet. Like her silent rituals at murder scenes and the meaning behind her father's watch. There was something about him that felt familiar which intrigued her, so she brushed the anomalies aside and just let him tag along for the cases.
It was Friday afternoon and they were winding down the week after having closed a case involving an art forgery scheme and an amnesiac witness.
"One has a lifetime of baggage and the other is going on a first date," Castle had remarked as they watched Jeremy and Emma depart the precinct hand and hand.
"Yeah," Beckett agreed wistfully.
The parallels to their own relationship were glaringly obvious. One with a memory of a full relationship and the other oblivious to their past.
"So you got any plans for the weekend?" Beckett asked casually, trying to change the subject.
"Hmmm… what day is tomorrow?"
"Just Saturday, the 3rd," she supplied, turning around to retreat into her office.
Castle racked his brain. September 3rd. Why did that date sound so familiar?
He vaguely recalled booking a venue and arranging catering for some sort of event on that particular day. And sending invitations to friends and family.
An image of save the date printed in fancy cursive on white cardstock flashed in his mind's eye.
Oh. Oh.
September 3rd.
Save the date
For the wedding of
Katherine Beckett and Richard Castle
The past six months of reacclimating to this alternate universe and learning about this new Beckett had made him completely forget what tomorrow was supposed to be. Marrying his best friend. The happiest day of his life in years.
The realization made his heart clench and sink to the bottom of his stomach. He had been doing so well coping with the fact that this Beckett wasn't his yet, but the hope that they would find their way back together gave him strength to go on.
It was a painful reminder of what he had lost in the old timeline. The love of his life.
"Uhh... no," he stammered. "No plans," Castle sighed dejectedly.
The change in his expression did not go unnoticed by the trained detective. When Beckett saw his demeanor shift, her eyebrows furrowed with concern.
"Everything okay, Castle?" she asked gently.
"Yeah. It's just… tomorrow was supposed to be an important day in my other timeline," Castle said with a tinge of sadness.
Oh, the parallel timeline thing. It must have been something pretty significant if he was this upset over missing it.
"Oh yeah? Is it something you could do in this one?" she offered.
"Uhh, not exactly."
"What was it?"
Castle shifted uncomfortably from foot to foot, contemplating how much he should reveal, choosing his words carefully.
"Remember how I said we were together in my other timeline? Like together, together?"
Beckett nodded. In some of his ramblings, Castle had implied they were apparently dating in his alternate universe. He never quite fully elaborated the extent of their relationship for fear of scaring her off, so Beckett just chalked it up to his overactive imagination and shrugged it off.
"Well, tomorrow was kind of supposed to be a special day for us," Castle explained vaguely.
"Like an anniversary?" Kate's inferred.
"Ah, something like that," he mumbled.
"Oh yeah?" Kate's lips quirked upward. "Which one? Gold? Pearl?" she played along, trying to lighten the mood.
"Not quite."
"Then how long were we together, Castle?" Kate teased.
"Two years, officially. But we worked together and knew each other for four before that."
"So, second anniversary," Kate remarked. "That's sweet."
Castle shook his head. "Well, no. Tomorrow's not the day when we got together," he corrected.
"No? I thought you said it was an anniversary."
"Yeah, it's supposed to be."
"Then what is it?" she probed.
Castle exhaled, lifting his eyes to meet hers.
"Tomorrow's supposed to be our wedding day," he finally revealed in a quiet voice.
Oh. Oh. Their wedding day? They were that serious? Shit, now she felt bad for teasing him about them. That explains why he was so distraught.
"Oh Castle, I'm sorry…" Kate tried.
"It's fine," he said as his gaze dropped back to the floor. He could feel his eyes pricking with tears. "I'm just going to head out," Castle excused himself quickly, wanting to be alone to mope.
"Have a nice weekend Beckett."
Kate squirmed at her desk as she watched Castle scurry out of her office after dropping that bombshell of information. She knew he liked her and could even possibly entertain the idea of them being together, but a wedding? Marriage? The way he looked so genuinely crestfallen at the idea of missing that milestone told her that he was serious about the whole alternate timeline thing.
She needed a way to fix this and at least apologize for poking fun at their hypothetical relationship.
Twenty-four hours later, Kate stood nervously at Castle's door holding a peace offering in hand. I shouldn't be here, the self-doubt said in her head, but it was too late. She had already knocked and now had to follow through with her humble attempt at an apology. She shouldn't even know where he lived, but she had looked up his address in the paperwork he signed as part of the deal to volunteer at the precinct, and now here she was, standing outside his door holding a cake and her pride close to her chest.
Kate continued to silently argue with herself mentally until she heard the door unlock and found herself struck by a pair of familiar blue eyes.
"Hey Kate," he greeted, surprised by the guest.
"Hey," she breathed. The way her first name sounded on his lips made her heart skip a beat.
"What are you doing here?" he asked.
"I, uhh, brought you something," Kate mumbled as she lifted the tuxedo cake up in supplication.
Castle tilted his head and looked at her curiously.
"Happy zeroth anniversary?" she tried meekly.
Nervousness churned in her stomach, fearing how he would react, until she saw a smile stretching across his lips.
"Is this about what I said yesterday?"
Kate nodded with sympathy.
"You looked so sad when you told me what today was supposed to be, so I wanted to do something about it. I know it doesn't make up for a wedding, but I was thinking maybe this could be the start of something new?" she said hopefully.
The grin on his face gave her butterflies in her stomach and made her heart melt a little.
"I'd really like that."
"Good," she felt a wave of relief wash over her. "Now how about we share this cake and you tell me what our wedding was supposed to be like?" she suggested, trying to relieve the tension.
The boldness of this Kate was novel to him, but he was grateful for the thoughtful gesture.
"Sounds great." He stepped aside to let her past the threshold and motioned to welcome her in.
Castle accepted the pastry box from her hands and peeked inside.
"Too bad it's not a wedding cake," he joked.
"I thought about it, but decided it might be a bit too insensitive," Kate teased.
"Probably a good call," he laughed as he led them to the kitchen.
Despite this not turning out to be the day he had dreamed about, Castle had the feeling this was going to be an important one for the two of them anyway. Fate had a funny way of bringing soulmates together.
For the first time in this new universe, he was excited about their future. If this was going to be anything like the first time he fell in love with her, Castle knew he was going to enjoy every minute of it.