Just before we start, a little bit of context/explanation.
This takes place after the finale. But, we're going to assume that because Kate and Rick were dating, she listened to him about letting it go, Montgomery never died, and Beckett never got shot.
And it has come to my attention that America celebrates Father's day in June! What trickery is this? Father's day is surely celebrated in September. At least in Australia. So, if you're wondering 'why on Earth is she putting up a Father's day special two and a half months late?', it's because, well... we celebrate Father's day on the first Sunday of September. So that would be today.

The wake up came at five in the morning. Groaning, Castle reached over to his bedside table and glared at the name on the screen. He loved her, honestly, he did, but why couldn't she understand that five in the morning was just a little too early for him.

"Castle," he answered, yawning.

"Sorry, did I wake you up?" Beckett asked mischievously.

"You know you woke me up," he said. "What's up?"

"There's been a murder," she replied. "I think you'll appreciate this one."

Twenty-four minutes later, Castle stood with Beckett at the back of a giant church hall, each clutching a steaming cup of coffee and talking with uniforms about various pieces of evidence.. Ryan and Esposito, who had also been dragged out of bed at this ungodly hour, were talking to the patrons of the church, who had arrived about five hours too early for mass to have their church council meeting and had instead found the dead body of the chaplain.

"Thanks, Aaronson," Beckett sighed, pushing her hair back. "Can you guys take inventory? Make sure you get everything – I want to know about every missing candle holders, candlesticks, anything, got it?"

"Of course, detective," he nodded, heading off.

"Candle holders?" Rick asked. "What, you think Professor Plum did it?"

"If there's a Professor Plum in the usual congregation, I'll give you a chocolate," she promised. "No, I don't think Professor Plum did it. But candle holders are very useful weapons. You can stab people with them, hit people with them… and, you know, churches are full of candles, Castle. Let's go talk to Lanie."

"You know, there are better things I could be doing right now," Lanie told them as she looked at a bruise on the victim's neck.

"Like sleeping," Castle agreed, looking pointedly at Kate.

"Oh, shut up, you," she said, rolling her eyes before turning her attention back Lanie. "What do you got for me?"

"Stabbed in the stomach," Lanie smiled grimly. "Not before he was beaten though. Unable to determine whether the perp used their hands or a weapon, but we can probably find out later when we forensics does particulates."

"Do you know what he was stabbed with?"

"Something skinny," Lanie said. "Skewer shaped."

"Sort of like the point of a candle stick holder?" Kate asked, looking sideways at Castle.

"That could do it," she agreed.

"Stop being right," he said to her. "It's too early in the morning."

Lanie stood up and looked between the two of them. "You know, I don't understand why writer here isn't quivering with excitement at the irony of this."

"It's half past five on a Sunday, Lanie. I'm not entirely coherent yet."

"Seriously?" Kate asked. "You haven't got it at all?"

"Alright, what is so terribly ironic about this case?" he asked.

"Come on, Castle, a Catholic priest has been killed. A Father, murdered on father's day?"

"Oh," Castle grinned, excitement seeping in to his features. "This is so cool!"

They'd just reached the street outside the church when Beckett's phone rang. She made a face at the screen. "Alexis," she said, holding the phone to her ear. "What's wrong?"

"Do you know where dad is? He's not home and he's not answering his phone. Is he with you?"

"What's wrong?" Castle asked, holding his hand out for the phone. "Tell meee!"

"Yeah, he's here," she said. "Castle, where's your phone?"

He checked his pockets and spun around, almost as though he would find it hanging on a string behind him.

"I've left it at home."

"He's left his phone at home, Alexis," Kate said. "Why do you ask?"

"Gram and I made him breakfast in bed, and then when we opened the door, he wasn't there!"

"Here, he wants to talk to you!"

She handed the phone to him and walked a little way down the street, Castle lagging a little way behind. She'd forgotten it was father's day until Montgomery had mentioned it to her the night before, asking if she'd made any plans. She hadn't, and she hadn't thought twice about calling Castle to the scene on his day of the year.

"Everything okay?" she asked him as he jogged to catch up and handed the phone back.

"Yeah. They were a little disappointed I wasn't there. I completely forgot it was father's day, so I didn't leave a note."

"You have any plans?"

"Not really," he shrugged. "Well, I didn't. Now we're solving a murder."

"You weren't going to go out for dinner?"

"We don't really do much for father's day," he replied. "What about you? Any plans?"

Find out who killed Father Charles," she shrugged. "I might invite my dad out for dinner."

"Father Charles?" Castle asked. "Seriously?"

"Seriously."

"That's a really cool name. So what do we do now?"

"We have to interview the congregation," she sighed. "But we have to wait until they actually get here. Ryan's getting a list of the usual attendees."

"So… breakfast?" he asked.

"Sure," she smiled.

Finding the killer of the chaplain wasn't too hard. As soon as they'd announced that they'd be interviewing the members of the church, a gangly kid with really bad acne that was sitting in the front row started sweating and looking around nervously, until finally he stood up, his hands above his head and confessed that he'd killed the priest because he didn't want to go to Sunday School that day.

With no other cases that day, the two of them went back to the loft and had the breakfast that Martha and Alexis had prepared earlier for lunch. Kate called her father and invited him over for father's day dinner with the Castle's, before they went down to the shops to buy ingredients for a roast.

"Happy father's day, dad," Kate said, waving as the door of the loft closed behind him. Alexis had gone up to her room an hour ago, and Martha had retreated now that there were no guests, so it was just the two of them, Kate and Rick, standing in the hallway.

"Thanks for dinner," Kate said sleepily into his shoulder as he steered them towards his bedroom. "It was great."

"I think we've established by now that everything I cook is fantastic."

She rolled her eyes, remembering with disgust the time he'd decided to try and make her eat a steak wrapped in a crepe with salad. "Yes, you keep telling yourself that," she replied.

"Thanks for inviting your dad over," he said. "A family father's day dinner," he grinned. "It made me feel a bit better. You know, I never really celebrated father's day before Alexis was born."

"I would understand the appropriateness of that," she replied.

"Just like how you don't celebrate mother's day anymore?"

"Excuse me," she said, her mouth open in mock anger. "I do celebrate mother's day. I go visit my mother at the cemetery. Just because I didn't invite you over to have dinner with her…"

"Sorry," he replied, sitting down on the bed. He grabbed her hand and held her in front of him. "You're beautiful."

"I have to tell you something," she replied.

"I'll take that as a thank you," he shrugged.

"No, I have to tell you something important, and I really, really don't want you to be angry."

"Why would I be angry?" he asked, rubbing a small circle on her hand.

She took a shaky breath and closed her eyes. Here goes nothing, she thought. "Rick… I'm pregnant."

His eyes widened as the words turned over in his head. The meaning didn't take long to sink in, but the surprise of the words rendered him speechless.

Kate, taking this the wrong way, started rambling. "I've known for a few days now, and I've been wanting to tell you and Lanie said I should wait for the right time, but there hasn't been a good time and –"

"Shh," he said, placing a finger to her lips. "You know what?"

"What?" she said, looking down.

He forced her chin upwards and locked eyes with her. "Best father's day present ever," he smiled.

Uh yeah...
Uhm... what did you think?