Page 19 of Her Last Vow

“Can I help you?” he asked, eyeing the access cards on the lanyards around their necks.

“Yes, we’re looking for Dr. Kline.”

“Oh, she’s in Lab 2A. Keep walking back and it’s the first lab on the right.”

They followed these instructions and, just ten seconds later, found Kline in an open-walled lab. She was positioned behind a standing desk; three monitors were arranged in a shallow arc in front of her, each one filled with graphs and a streaming series of numbers. Kline turned when Rachel knocked on the thin frame of the entryway.

"Agents! Dr. Sloan just rang to let me know you were coming.”

Of course he did, Rachel thought.

Kline waved them in and gestured at two chairs pulled up near the desk. They were plain and utilitarian compared to the furniture upstairs. She clapped her hands twice, excitedly and smiled widely at them. “So you need to know about the model, right?”

Taken slightly aback at Kline’s energetic personality, Rachel could only nod. Because honestly, once the technical jargon got too complicated, she wasn’t sure what in the hell they needed to hear about.

“Yes, please,” Novak said. Apparently, he was in the same boat.

"I have to say, I've been hoping somebody would ask me about the model directly instead of just asking Nina to explain it secondhand."

"We'd like that," Rachel said, sitting. "Can you walk us through how it actually works? Not the marketing version."

Kline's face lit up in a way that made her look younger than she probably was. "Finally. Okay." She turned one of the monitors toward them, pulling up a chart with a curve rising and then gently bending downward. "This is a composite health trajectory. Every user gets one of these, built off dozens of individual inputs. Resting heart rate variability, VO2 max, sleep architecture, recovery windows, even grip tremor. We don't just track where those numbers are. We track the rate of change in each one over time."

"And that gives you the peak date," Novak said.

"Right. Well, mostly. See, it's not one input predicting one date. It's a convergence model." Kline pulled up a second graph layered with multiple colored lines. "Imagine six or seven different curves, each one representing a different biological system, each one rising and eventually declining at its own pace. The peak isn't when any single line is at its highest point. It's the moment where the combined slopes cross a threshold where the whole system is operating at maximum efficiency, all at once. After that point, one or more of those systems start to decline faster than the others compensate for."

"How far in advance can you predict it?" Rachel asked.

"For a user who's been wearing the device consistently for at least a year, we can usually narrow it down to a window of about ten days…about eight to twelve weeks out. As we get closer, the model recalibrates weekly, and the window tightens. By the final month, we're often within two or three days." Kline said this with no hesitation, no attempt to soften it. "It's genuinely one of the best predictive models of its kind. I built most of the recalibration logic myself."

"You sound proud of it," Novak said.

"I am proud of it," Kline said simply. "It's good work. I don't think there's anything wrong with saying that. As a matter of fact, I’ve turned down ridiculous offers for the secrets and to jump ship to other teams.”

Rachel let her explain a little further, watching the way Kline moved between the monitors. She pulled up sample datasets and narrated the mechanics behind each one without being asked to do so. There was nothing evasive in it, none of the careful phrasing Oakley had used or the philosophical hedging Sloan had leaned on. She answered as if the questions themselves were satisfying. Rachel got the very real sense that Kline would be more than happy to speak with them for hours about this if her schedule allowed for it.

"Can I ask you something else?" Rachel said once Kline had paused. "Something about the team, not the algorithm."

"Sure."

"How many people work on this model with you?"

"Eight of us total on the model team, including me." Kline shrugged. "Data scientists, a couple of ML engineers, one statistician who mostly checks our work. We’re a small group, but tight-knit."

"How would you describe the mental health of your team?" Rachel asked. "Any concerns, past or present? Anyone who's struggled, taken leave, anything like that?"

Kline laughed, a short surprised sound. "You're asking if one of my engineers is secretly murdering people."

"No. I'm asking about your team's mental health," Rachel said, keeping her tone level. She didn’t bother asking how she knew they were here discussing murders. She thought it was a very safe assumption that Sloan had told her during their brief call.

Kline looked at her for a second longer, waiting for the joke to reveal itself, and when it didn't, something in her face shifted. The easy warmth from a moment earlier folded into something tighter.

"You're serious," she said.

"I am."

"That's..." Kline sat back slightly, arms crossing. "That's insulting, honestly. These are some of the most careful, disciplined people I've ever worked with. We built something that's actually helping people, and now you're sitting in my office asking if one of them is a killer because they happen to understand the math."