In Time of Our Lives, Beckett is the one taken to the AU universe, in which Castle is doing great both professionally and personally (probably has a family and kids)
She goes home, because nowhere else feels safe in this odd, alternate version of her world.
At the precinct, people had addressed her as ‘Captain’ and no one had ever heard of her partner. Richard Castle? Ain’t that some Pulitzer author or something? Esposito had asked when she had questioned the boys on whether or not Castle had returned from the coal factory.
Worry had churned in her gut when she had woken alone on the cold factory floor, no sign of her fiancé, but she had assumed he would have gone back to the precinct after deciding to abandon her for whatever reason. But he’s not there and no one can tell her anything about the man they’re all acting as if they don’t know. So she goes home, pleading to the universe that’s seemed to have played a cruel trick on her that he’ll open the door and everything will be okay again.
She nearly panics when her key fails to fit in the lock, practically beats on the surface of the door in result, in her haste to see him, to know.
“Coming,” she hears the bellow of a familiar voice and her heart leaps in her throat because it’s him. Castle.
The door swings open and there he is, safe and sound, and she doesn’t hesitate in lurching forward, wrapping her arms around his neck.
“Thank god,” she whispers, using the advantage of her heels to press her cheek against his while she inhales the comforting scent of his aftershave.
Castle’s hands touch her waist, his hands curling around her hips, and - and he’s pulling her away from him.
“Um, I’m sorry, but I think you’ve got the wrong apartment,” he informs her, dropping his hands from her waist and carefully removing her arms from his neck.
The relief turns to a ball of stone cold dread in the pit of her stomach.
“What are you talking about? Castle, I - is this some kind of prank?” Her heart pounds with hope. Maybe, maybe he’s just finally getting her back for his April Fool’s themed birthday party she threw him last year.
His brow furrows, a touch of concern coloring his irises. “I’m sorry, Miss. But I’ve never met you before.”
“Cut it out,” she demands, refusing to acknowledge the horror swirling through her bloodstream, but when he still remains looking truly, genuinely confused, she becomes a little desperate. “We were just in bed together this morning-”
Castle whips around and Kate glances past his shoulder to see the pretty blonde woman standing in his living room, a furious expression claiming her wide eyes and high cheekbones.
“Who is she?”
Both women blurt the question at the same exact moment and Castle glances between them almost comically, gaping like a fish out of water.
"I have no idea who this woman is,” Rick scrambles, turning his back on her and towards the other woman. “Cara, I don’t know her, she just showed up, I swear-”
“This is the last straw, Richard Castle,” the other woman spits out, spinning on her heel and stomping up the stairs. “I’m taking Andrew and I’m going to my mother’s until you learn how to keep it in your pants.”
“Cara, no, wait.” Rick bounds after the blonde and Kate can hear the yelling on the second floor, the cursing, the cry of a baby.
She has to press a hand to her chest, her breath coming in short, sharp bursts, and oh god, she can’t have a panic attack, not now.
Kate barely notices the woman and baby storming past her in the still open doorway, barely hears Rick shouting after the two, doesn’t realize he’s channeled his anger towards her until she’s crumbling to the floor and her vision has gone white.
She opens her eyes to see him glaring at her from the armchair across the living room, his arms crossed tightly over his chest, his mouth set in a strict line. Her Castle would never look at her that way.
Night has fallen in the parallel universe and she wonders how long she’s been here, lying unconscious on the unfamiliar version of this man’s furniture.
“Thank you,” she murmurs, sitting up slowly on the leather sofa. “For moving me to the couch.”
“Couldn’t just leave you on the hallway floor,” he retorts, so cold, so angry, and she almost starts to cry from sheer frustration.
She wants to go home, she wants her partner back.
“Who do you think you are?” he seethes suddenly. “Coming here and disrupting my life, my family? My girlfriend took my son to her mother’s and won’t answer any of my calls because she thinks I cheated on her with you.”
"You’re not supposed to be with her,” she whispers, pulling her knees to her chest, digging her quivering chin in between the bones. “This isn’t how your life is supposed to be, Rick.”
“I’m not supposed to be happy?” he bites, but there’s no real malice in the words, and she wonders how happy he really is. He may be succeeding in his career according to her colleagues at the precinct, may have attained what some would consider the picture perfect family, but success doesn’t equal happiness.
“I make you happy.”
He scoffs. “You’ve made me quite the opposite of happy.”
“Will you give me a chance to explain?” she tries, hoping a less forceful approach might help. He may be different from her version of Castle, but this is still essentially the same man, and she can’t see Richard Castle turning down a good story in any universe.
“Why not?” he mutters, slouching in the armchair and allowing her to tell the story of how she came to exist in the wrong universe.
“So I do, make you happy, I mean,” she finishes, holding her breath as she waits for his response. He listened to her throughout her entire recounting of their most recent case, about the artifact she’s narrowed down as her purpose for being here, and he even sat through her stories about her life with him in another world, but his poker face is better in this universe and she can’t read what he’s thinking.
“In this alternative universe of yours,” he sighs, scrubbing a hand over his eyes and pushing his hair back from his forehead. “I like science fiction, Kate, I even believe in certain aspects of it, but this doesn’t feel like some sign from the universe that I’m missing out on something other than what I have.”
“You were happy with her then?” she hisses, unable to help the bitterness at seeing him with another woman. “You’re happy churning out novels that are boring as hell to read?”
His jaw tenses, but he doesn’t move to deny her, to refute her claims about books she’s never even read, and part of it makes her ache, to know he isn’t as happy as he deserves to be, but the rest of her flares with hope.
“In my world, the Richard Castle I know would not settle for anything less than extraordinary.”
“What makes you so extraordinary, Kate?” he asks, not angry this time, just worn.
She reaches for him for the first time, grateful when he allows her to cover his hand with her own. It eases the splitting ache in her chest to share at least one small form of connection with him, even if it is probably the only one she’ll be allowed in this world.