Thanks to Jill (jstar1832), encantandaa, and Jan for all their help and support!

On their way from the apartment complex to the precinct

Castle and Beckett had finished looking through Sarah Manning's bag and were on their way to the precinct. Once they arrived, she would have had the unfortunate job of calling the victim's parents to inform them of her death.

"Do you always have to call the family?" Castle asked, glancing at her. It was definitely a job he would never want to have – to tell someone that the most important person in their life had their own life taken from them.

"Sometimes – sometimes another detective does it. Like if the family is local uniforms will just go to tell them in person," Kate answered as they pulled out into traffic city traffic.

"That must be hard for you. Even after you've done it a few times." Castle was speaking quietly, deep in thought at the prospect of having to make such a call to unsuspecting parents and loved ones.

"It doesn't get any easier," Kate responded, trying not to sound too emotional, although this was oftentimes one of the worst parts of the job for her.

"I can't imagine getting that call. About Alexis I mean," he added, seeing the confused look that Beckett was giving him. Castle was a devoted father and would gladly die for his daughter if he ever had to. She had read this about him, but each time he mentioned her it took Kate by surprise.

He shifted in his seat, turning to stare out the window. From the corner of his eye, he saw Beckett looking at him, studying him carefully, but he ignored her. It was one of the first times he appeared unselfish and actually human to her.

"Well Castle, chances are you won't ever have to," Kate said, interrupting his thoughts.

He turned toward her, grinning, but her calming words and the reassuring smile on her face did nothing to erase the thoughts going through his head.

On their way from the precinct to the playground

Castle and Beckett got into her vehicle and were quickly on their way to the playground, hopefully to find Sarah's friend Chloe Richards. Kate knew she shouldn't have been fooled on their car ride earlier. Just for a moment, when talking about his daughter, Castle had dropped his guard, and he'd appeared unselfish and as if he were actually human. Now, though, he was back to his usual annoying, obnoxious self.

"Do you remember what I said Castle, right before we went into interrogation?" Kate asked, a twinge of annoyance in her voice.

"Something about an invitation? I'm not sure, I don't really remember." He was purposely being sarcastic and he smirked, avoiding her glance.

"I said you are here by invitation only. What does that mean to you?" she snapped.

"That I'm the guest of honor?"

"No, Castle. It means you are not there to ask the suspect questions; you are there to observe. And don't write down that I lied to the suspect! That's not exactly something I want to be public knowledge."

"Oh I wouldn't worry about that Beckett. Plus, I knew what I was doing." Her face didn't soften though so he eased up just a bit. "I didn't say anything wrong did I?"

"Yes, you were wrong by speaking at all!"

"How am I going to help you if you don't let me ask questions?"

"I don't need your help!" She was becoming exasperated and defensive, two things she did not want to be at that moment. The last thing Kate wanted him to see was this side of her – she had to stay composed and in control.

"I helped you on the last case!"

"Until you almost blew it by believing Harrison's alibi!"

"Ok, let's please just stop arguing and focus on the case? I just want to help…and observe."

Kate took a deep breath before responding. "Okay fine," she acquiesced. "We're here anyway." She pulled over as they approached the park, ready to look for Chloe Richards and hopefully find some answers.

From the playground to the office building

Castle and Beckett thanked Chloe then headed back to the car. They needed to visit Mr. Peterson and find out exactly what was going on. Were he and Sarah having an affair? And did that lead him, or someone else, to kill her?

"She was a nice girl, don't you think?" Castle inquired as they climbed into the car.

"Yeah I guess. A little odd…skittish," Kate responded. As a veteran detective, she was able to pick up on the little things that he apparently hadn't noticed.

"Wouldn't you be if you were being questioned by an officer?" he asked, surprised that she felt that way about Chloe, and also wondered exactly what it was that tipped her off.

"Just her tone of voice, the way she was looking at us. Little things," Beckett responded nonchalantly.

Castle looked at her, trying his hardest to remember the moments she was speaking of. "Don't worry Castle," Kate said with a little laugh. "I'm a trained detective remember? You're just a writer." She grinned at him, seemingly happy with herself for making the little dig.

"Oh I'm not worried Detective. But thank you for your concern." She rolled her eyes at him as they continued driving towards the office building.

"What I'm concerned with is why Mr. Peterson would be having an affair with his twenty-three year old nanny."

"Well, that's something I think I know the answer to…"

"Haven't you had enough affairs with the young and single Castle?"

"What do you mean?"

"All those young, single, mothers you were talking about," Beckett answered. "Maybe you should stop thinking about women not that much older than your daughter that way."

"Ok…ew…" That shut him up for a bit.

From the office building to the precinct

"Did you see what I did there? You know that 'don't leave town' line?" Castle smirked as they got back into the car.

"Oh yeah…I saw it." Kate couldn't hide her annoyance at him.

"What?! I thought it was a nice touch. See how much I'm learning from you?" Castle's cocky attitude was part of his charm, wasn't it?

Kate chose to ignore that statement, simply rolling her eyes and moving on. "So…he was having an affair. Hopefully with someone his own age."

"So he was having an affair. Maybe she killed Sarah!" Castle exclaimed excitedly, as if a lightbulb had just gone off in his mind. Of course he was kidding…half kidding.

"Why on earth would the woman Mr. Peterson was having an affair with kill Sarah Manning, Castle?"

"She thought he'd hit on her?" Castle suggested.

Kate signed. "Can we please stick to actual evidence? Please?"

"Okay fine," Castle conceded. "I wonder if Sarah knew about the affair, or just thought Mr. Peterson was actually…working late."

"Who knows. But the more cases I solve the more I lose faith in the male species."

Castle was silent for a few seconds, finally snapping out of his reverie and looking at Kate. "I'm sorry what?" he said. "I wasn't listening."

Kate threw her hand up as she proved her point.

From the precinct to see Mrs. Peterson

"So Mrs. Peterson lied, huh," Castle restated what they had just learned.

"That's what it looks like." Once again Castle and Beckett were on the go, this time to see Mrs. Peterson who had apparently lied about her alibi.

"Does that make her our prime suspect?" Castle asked.

"Well it definitely puts her up higher on the list. Why would she lie if she had nothing to hide?"

"Maybe she was having an affair, too."

"Well now, aren't they just one big happy family."

"See Beckett, this is what you're missing out on by not being married!" Castle said. "As soon as you get married, the men will be pouring in." Castle seemed to know a little too much about having an affair for her taste.

"Way to restore my faith in the male race, Castle."

"Hey, don't be mad at me. She's the one having the affair."

"We don't know that's what she was doing," Kate reminded him.

"Oh you're right….maybe she was just off you know…committing murder or something."

From the precinct to Chloe's apartment

"What are you thinking Detective?" Castle asked. He and Beckett seemed to have gotten a break in the case.

"I'm thinking that Chloe is the one that killed Sarah." She put the siren on as they sped off towards Chloe's apartment.

"Okay well, I get that Chloe followed Sarah into the laundry room, but what's the motive?"

"We'll figure that out once we get to her apartment."

"Any theories?" Castle asked.

"Jealousy? Maybe she wanted to have the affair with Mr. Harris, and since Sarah was, she couldn't."

"Not to say I hope you're right but…" Castle paused for a second before putting his finger on the volume button on the radio. "Do you mind if I raise this? The siren's kind of drowning out the music." All Kate could do was look at him and shake her head.

From Chloe's apartment to the Peterson's building

Kate pulled out her cell phone and dialed the precinct. She needed to keep Espo and Ryan up to date with details of the case.

"Are the boys meeting us at the apartment?" Castle asked excitedly. He was getting another opportunity to catch the killer; this time, hopefully, with his shoes on.

"Yeah they're on their way," Kate answered. "They'll get there first so hopefully Chloe will be on hold for me when I get there."

"Don't you mean for us?" he asked with a bat of his eyelids.

"Castle? Shut up."

"Got it. So this must be some special kind of crazy we have going on. Not just murder, but delusional murder."

"Yeah well, it happens," Kate said. "Obsession mixed with the right combination of mental unstableness can do dangerous things to a person's psyche. It can even cause them to kill."

Castle thought for a moment. "It's a good thing she hasn't gotten to Mrs. Harris," he finally added. "Have you seen something like this before Detective?"

"I've seen quite a lot, Castle. People kill for money, love, power, you name it. Sometimes, as it appears in this case, something just snaps."

Castle looked at Beckett, his eyes widening, thinking how grateful he was that his two ex-wives had never gone off the deep end…at least, not really.