Kate remembers the exact moment when it happened, or began to happen. It wasn't an inkling, really, just a slightly uncomfortable feeling. Something was fractionally off. She had written it off to her nervousness about working with the Federal task force and her apprehension about making a relationship work when Castle was in New York. She was digging herself deeper and deeper into that hellhole of a job in Washington and never had time to be with him. Yeah, that was one eminently erasable chapter in the book of her life.

But the little uncomfortable feeling wasn't about her job or the shambles into which it quickly disintegrated, nor was it about being separated from her fiancé. It wasn't about the brief if torturous period of unemployment that followed, either, because once she returned to her old job at the 12th, it got worse. The niggling sensation, the little buzzing in the back of her mind that she couldn't silence, wasn't about any of that. She couldn't take it apart to analyze it. All she came to know, over many months, was that this amoeba-like fear was about Alexis.

Kate and the young woman who is now her step-daughter have a pleasant if not close relationship. Their history is uneven, stained with painful episodes that were rooted in Alexis's invective, accusations that Kate had betrayed her father, or put him in unconscionable danger, lied to him or stomped on his heart. There was, as there always is, fault on both sides, but Kate likes to think that she and Alexis are now affectionately accepting of each other. It is a hard-won peace, but it seems genuine. And yet, and yet.

When Kate began her training in Washington, trapped in the capital's stifling summer weather and the even more suffocating restrictions of her new job, Alexis was in Costa Rica, enrolled in a study program about the rain forest. Castle complained to Kate about how difficult it was to reach his daughter, how they couldn't phone or Skype or email, because she was so deep in the jungle. He was prepared for her to return with photos and stories and mosquito bites; he was thoroughly unprepared for her to bring home a boyfriend. An unshaven, messy, irresponsible boyfriend who was unquestionably exchanging body fluids with her.

Castle, the doting, over-protective father, would almost certainly have objected to any boyfriend whom Alexis brought under his roof, but the key—at least to Kate, the ever-so-slightly questioning but silent Kate—was that this boyfriend, with the dubious name of Pi, arrived unannounced. Uninvited. Unintroduced. Probably unwashed. It had taken Castle two full days to learn the little gnat's last name.

Kate arrived in New York not long after Alexis and Pi. Suddenly she had nothing but time, and she filled it with nothing but anxiety, first about her professional life but then, bit by bit, about her personal one. For the first time in her adult life, she was truly not alone. She all but lived in the loft with Castle, Martha, Alexis and, regrettably, Pi. In addition to all his other un-'s, Pi was undocumented. According to Alexis, he had lost his passport and thus couldn't move on. That was ludicrous: he could have gotten a replacement virtually overnight. Kate couldn't understand why Castle didn't say something, especially because Pi irritated the crap out of him, but she wrote it off as his decision to accept his daughter's choices. Kate had to keep her counsel. Her relationship with Alexis then was fragile, and even a gentle question would be seen as an attack on Alexis's credibility as well as Pi's.

As time went by, Kate's unexpressed worry grew, and focussed excessively on Alexis. She just wasn't the same girl. She looked slightly different, a little puffier, but in her attitudes she was the opposite, steely and unyielding in even the most insignificant discussion. It was one thing not to give Kate an inch, but her father? Kate understood and empathized with adolescent rebellion; it was Alexis's other behavior, or lack of it, that unsettled her. Pi and Alexis were young, and supposedly madly in love. They should have had their hands all over each other, all the time, and yet they barely touched. She and Castle, not Alexis and Pi, were the ones who were caught with their pants down on the living room sofa, more than once. And when Alexis talked about Pi, how smart he was, how committed, there was no passion in her voice. Even when they broke up, without warning, the fracture seemed painless, at least on Alexis's part. No one heard how Pi felt; he simply disappeared. Kate was relieved and grateful when father and daughter patched things up, but she was still wary. And she couldn't say a word. Kate had long ago set the Olympic standard for steel-edged repression—tamp it down, bury it so deep no one will know it's there—and though she had worked hard to be completely open with Castle, this was one concern she wouldn't share. She hid it so well that even he, who was so finely tuned to her moods, didn't suspect that she suspected something.

After Castle was abducted last spring, on his way to their wedding, she and Alexis could have bonded in grief and fear, but they didn't. Not really. Kate was too consumed with finding him, with holding herself together enough to search every day and night, to spend much time with anyone or on anything else. And when Kate expressed her barely-contained rage that Castle had no explanation for why he hadn't been in touch with her for two agonizing months, Alexis flailed at her again. Eventually, they made peace again, and it was holding. Until today.

Today the tiny canker that Kate had picked and picked and picked at, off and on for a year and a half, the little sore that she hidden so skillfully, came blazing to the surface. And just as she could pinpoint when her worry began, she could fix to the second when that worry escalated to full-on panic. She realizes now that there had been an early-warning bell this morning, when she and Castle were having coffee. Alexis was leaving for the library, and on her way out she kissed Castle—and Kate—on the cheek.

"A goodbye kiss? When did that start?" Castle asked.

"Just now. Yeah, no, I'm as surprised as you are," Kate said, though what went through her mind was, "Surprised? Hell, no. Shocked." Before she could give a moment to reflect on Alexis's action and her reaction to it, she got the call: a body drop.

A young woman had been strangled and left in a dumpster, but what first appeared to be a tragic if straightforward case quickly turned into something else. Body doubles were involved, as they had been in that traumatic case last year, and it was clear that Dr. Kelly Nieman was also involved in this one. Kate was already having trouble staying in her skin when Ryan approached her and Captain Gates. "We looked into Kelly Nieman," he said. "She has a surgical clinic in Costa Rica, but she also began seeing patients right here in the city six months ago."

Kate manages to keep the roiling fear at bay as she tells the Captain that she will call on Doctor Nieman herself. After collecting her bag and her wits, she takes the elevator to the garage. Her hands are shaking so badly now that she can barely open the door. She gets in, sits down and places her forehead on the cool steering wheel, fighting to control her breathing. Auden, that Auden poem, that's it. Her mind becomes a meeting place for fire and ice as she remembers a few lines from Auden. How did it go? "Perhaps that mysterious noise at the the back of the brain…" What was the mysterious noise what, what, what? Yes, she has it now: "this Horror starting already to scratch its way in."

Kate is almost convulsing now as she thinks: the Alexis who came back from Costa Rica last year wasn't just different. She wasn't Alexis at all. She was another girl altogether. She was the creation of Kelly Nieman.

The Horror has scratched its way in. Oh, God.

TBC…