Zoe eyed his hands and spotted the telltale shake. He was no doubt anxious to get out of FBI custody and go home for a drink. He’d been with them for hours now, and he was a serious alcoholic. The ratio in his bloodstream was decreasing, leaving behind the physical symptoms he would no doubt do anything to avoid.

Zoe had done anything but forget about Wardenford. During the drive back to HQ, she had formulated a plan. Shelley would go to the forensics lab and encourage a rush on the hair that they had found, using her natural charm to get it done quicker than Zoe could. Meanwhile, Zoe would talk to their former suspect.

Maybe it was obvious now that he was innocent of being the killer, but that didn’t mean they needed to let him go right away. He had been able to glean something, at least, from the equations—and he had spotted Zoe’s abilities right away. That meant that, for now at least, he was an asset.

An asset who could help them with this latest piece of the puzzle.

“Take a look at these,” Zoe said, dropping the photographs in front of him and taking her seat.

She was banking on the fact that Wardenford would be distracted enough by the allure of the mathematical puzzle to not notice that he had now been proven innocent. Just as she herself would not be able to resist attempting to work it out. True to form, he snatched up the pictures immediately, his lips moving silently as his eyes traced over the new equation.

Zoe watched him carefully as she had before. There was still no flicker of recognition, not that she could see; only eagerness to take on a challenge. She had harbored the small suspicion that Wardenford could still have been involved, with an accomplice taking down North, but now that was gone. His reaction coupled with the shaking of his hands, which were not steady enough to tackle a victim or write out a clear equation, told her everything that she needed to know.

Dr. Edwin North’s family and colleagues may hold more answers. Shelley would move on to talking with them after she had visited the lab, but Zoe wanted to be here. Working on this. She still felt that this was the most important part of it all—that putting the equations together might reveal a larger solution, something that required lengthy workings and complex enough math to stump even the experts.

Even Zoe, until, she hoped, enough was revealed to facilitate that breakthrough.

The only sound in the interrogation room was the ticking of the clock above the door and a slight shuffle of papers now and then, as Zoe and Wardenford both studied copies of the photographs in silence. The equation was just as before: seeming to make sense up to a point, then disintegrating into nonsense. There was a mismatch somewhere, something that did not fit.

“It’s wrong,” Wardenford eventually declared, planting his hands firmly onto the tabletop to hide their shaking. “Just the same as the other two. The last part is broken.”

Zoe had already reached the same conclusion, but there was something about what he said that drew her attention. “The last part?”

“Yes, the final three lines. Look at them—they’re totally unbalanced against the rest of it. This one even switches to different symbols. Where is N in those lines? The first section seems weighted towards using N as a crucial part of the equations, where it does not appear at all in the end part.”

Zoe cast her eyes over the equation again, though her memory had already told her he was right. The last three lines… was there something in that?

Seized by a sudden inspiration, she flipped back through her notebook to where she had written out the first two equations. “There must be a connection between all three,” she said.

“That’s a false equivalency,” Wardenford shook his head. “Just because the same person wrote the three equations on bodies in the same way, does not necessarily mean that they are part of the same overarching equation or connected in a further way.”

Zoe could not listen to him. How could she? If he was right, then there was no way to solve the equations. And if there was no way to solve them, then there was no extra clue hiding in there which would help her to link the three victims and trace the link back to the killer.

There had to be some kind of connection.

There just had to be.

“You’re wasting your time,” Wardenford insisted, but Zoe was no longer hearing him. She started to scribble out the last three lines of each of the equations on the back of one of the photographs, in order. Just the last three lines, the three that didn’t make any sense in each of the cases.

When she was done, she stopped and looked at it. It made a full equation in itself, and now the signs were starting to make sense. This was something that she could understand, at last. This was something—somehow—familiar?

Wardenford reached for the paper and spun it around so that he could read it, his eyes flashing from left to right over and over. It was beginning to dawn on Zoe exactly why that equation looked familiar, something rushing through the synapses in her brain to tell her just where she had seen it before—

And, oh. Oh no.

“I’ve seen this before,” Wardenford said, even as Zoe’s mouth opened to cut him off, to tell him to stop. “It’s a theoretical equation that a local mathematician came up with. It made quite a stir, actually. Her name was something—what was it now? Apple… Applewhite. Dr. Applewhite, that was it. This is her equation, in full.”

Zoe knew now what she had done. It was clear. She had been desperate for a way to make sense of it all, and so she had fallen back on something that she recognized. Just like how other people supposedly saw a face on the moon, instead of measurable craters and hills and valleys. There was no face on the moon.

In just the same sense, there was no way that Dr. Applewhite really had anything to do with this.

It couldn’t be right—it was all just a coincidence. Maybe Zoe had even copied out the equations incorrectly. She flipped back in her notebook, checking and rechecking.

“That’s your culprit, then,” Wardenford pronounced, leaning back in his chair and folding his arms at ninety-degree angles across a puffed-up chest. “Dr. Applewhite. She’s got offices somewhere nearby, does studies on people with abilities like yours. Hang on, you probably know her, don’t you? She must have finally cracked.”

Zoe’s mind was racing, trying to find a possibility which explained all of this. Coincidences happened, even if they were not statistically likely. In fact, that’s all they were: the collision of things that were somewhat likely, happening in an order which was less likely and yet still possible. In an infinite universe, everything that was possible to happen would happen. That was the theory, wasn’t it?

“This cannot be anything to do with Dr. Applewhite,” Zoe blurted out abruptly, pushing all of the photographs together into a messy pile that she could scoop up into her arms. “You are no longer a suspect, Mr. Wardenford. You are free to go. See them at the front desk about getting a taxi.”

She rushed out of the room, opening the door awkwardly with one hand holding the bundle of images against her chest, and almost collided with Shelley in the corridor.

“In here,” Shelley said, her voice harder and flatter than Zoe had ever heard it before. She barely had time to register what was going on before they were both sealed away in the observation room adjoining the interrogation room, where on the other side of the black glass James Wardenford was getting up to leave.

“How much of that did you hear?” Zoe asked, hating the tremor in her voice as she asked it. Hating the fact that there was something she hadn’t wanted anyone to hear at all.

“More than enough,” Shelley said, shaking her head. “Zoe, there’s something else you need to know. Forensics already came back on those hair follicles. They didn’t get a match in our database.”

“That does not mean anything,” Zoe pointed out. “Only that our suspect has not been previously arrested. We will be able to find a suspect eventually, and then we can test them against the hairs.”

“We already have a suspect,” Shelley said. Her voice was low and soft, but Zoe still flinched away when Shelley reached out to put a hand on her upper arm. “Z, we have to follow through on this lead. You know we do. We have a professional obligation.”

“There is no lead,” Zoe snapped. “I simply wrote it down wrong. I will go back to our files and work out where I went wrong. There is absolutely no real connection here. Taking a sample slice out of the equations—you could make them resemble anything, if you wanted to.”

“I know you don’t want to see it,” Shelley said. Her tone was still soothing, but there was a determination in her eyes that Zoe understood fully. There was no getting away from this. “Call Dr. Applewhite and find out where she is. We have a responsibility to ask her to submit to a DNA test.”

“It will not show anything. She is not connected, not in any way,” Zoe argued hopelessly. She knew that Shelley was right. She wouldn’t even be able to submit paperwork omitting this without risking her job. She could even go to court for withholding something this serious.

“Then she will be ruled out. But, Z, you should prepare yourself.” Shelley gave her a stern look. “We have to obtain a DNA sample from Dr. Applewhite. And if it matches, we will have to arrest her for murder.”

CHAPTER SEVENTEEN

Zoe had a sick ache in the pit of her stomach. She couldn’t tell whether she was about to throw up, lie down and die, or give birth to some kind of monstrous child. The feeling had been growing by the second since Shelley had laid down the law, and now it was threatening to totally consume her.

Zoe had had no intention of implicating anyone, especially not her beloved mentor. She could see clearly that it had all been her own mistake. There was no connection—truly, none at all.

She just couldn’t get Shelley to see that.

Ultimately, it didn’t matter what either of them believed. The wheels were in motion now, and procedure dictated that they follow every possible lead. If they were found not to have followed this through later, they could both lose their jobs—and it could even jeopardize the case once they brought the real killer to trial. Defense lawyers loved nothing more than loose ends.

Zoe didn’t need to call Dr. Applewhite to know where she would be: in her office, as she was every day at this time. Likely meeting with someone from her case study group. She was a busy woman, and this interruption to her working hours would no doubt cause her no end of hassle. Zoe felt guilty even to be bringing this to her door. With every minute that passed, she was thinking up another reason why this was quite possibly the worst thing that she had ever done.

“How can I help you?” the bespectacled receptionist in the cool white room that served as Dr. Applewhite’s foyer asked them, if not with suspicion, then certainly with curiosity. She must have known that no one was due to come in at that moment.

“We need to speak with Dr. Applewhite,” Zoe said, feeling bile rise in her throat as she said the words.

“She’s occupied at the moment,” the receptionist said, glancing up to check the clock. “She’ll be out in about twenty-five minutes, I should think.”

Shelley took out her badge and laid it on the desk for a moment, keeping her fingertips in contact. “It’s rather urgent,” she said.

The receptionist’s mouth formed a shocked “oh” of nude lipstick, and she was reaching for an internal phone when Zoe stopped her.

“We will wait,” she said, gesturing to the nearby chairs for Shelley’s benefit. The last thing that she wanted to do was to embarrass Dr. Applewhite by bursting in on something scheduled. Especially given that this was nothing at all to do with her, and only Zoe’s own mistake. Dr. Applewhite could not be anything but innocent. There was not even the shadow of a doubt in Zoe’s mind that this was all about to be cleared up, albeit painfully and awkwardly. It was that part that she was dreading.

The minutes ticked by interminably. Even for Zoe, who normally had an impeccable inner clock, it seemed to drag on for hours. Then there were only ten minutes to go, and she began to really sweat about what was to come. As soon as that happened, time altered itself again: ten minutes flashed by so fast that Zoe had to double-check her watch when Dr. Applewhite’s door opened.

“Great work. I’ll see you next week,” Dr. Applewhite was saying, ushering a young man out into the waiting room. He ambled past them with a curious look, even glancing back over his shoulder as he heard Dr. Applewhite greeting her former charge.

“Zoe! What brings you here? More news on the case already?”

Zoe could barely look her in the eye as she got up from her seat, nodding her head. “There has been an update. We have a new body, with a new equation.”

“Do you have a copy for me to look at?” Dr. Applewhite asked. Her head was swinging from Zoe to Shelley, no doubt confused by their unhappy expressions. It was like a pendulum in a clock, ticking onward almost at an even rate of seconds. Tick. Tick. Tick.

“It would be better if we could talk to you in a more private setting,” Shelley said tactfully.

“We can use my office.” Dr. Applewhite gestured back toward the open door, and even took a few steps back before Shelley interrupted her.

“No. We’d be better off in our office, so to speak. We need to ask you to provide a voluntary DNA sample.”

Dr. Applewhite paused, looking over at Zoe. Zoe looked up and met her eyes, and instantly wished that she hadn’t.

“What is this about?” Dr. Applewhite asked, her tone less sure now.

“We need to eliminate you from the case,” Shelley said, simply.

Dr. Applewhite was still looking to Zoe, as if waiting for confirmation. All she could do was give a single, sharp nod, the shame weighing heavy on the back of her neck.

“All right,” Dr. Applewhite conceded, uncertainty flooding her voice. She glanced over to her open-mouthed receptionist and nodded to her, receiving a nod in return as the other woman began shuffling through an appointment book.

Zoe allowed Dr. Applewhite to walk out of the office first, Shelley behind, with Zoe trailing last. This was the last thing that she wanted. She just hoped that it would be over quickly, so that she could apologize and make it right.

***

Zoe watched uncomfortably as Dr. Applewhite held her mouth open for a swab, through the glass window of a door in the lab area of the J. Edgar Hoover Building.

“I do not like this,” she muttered, just loud enough for Shelley to hear her.

“I know you don’t,” Shelley said, holding back what Zoe imagined was the internal you’ve made that pretty clear. “Let’s just hope that it clears her and we can move on to some other angle.”

Zoe gritted her teeth, keeping her mouth shut. Shelley was right. That was all they could do, now; wait for the results and hope.

“All done.” The lab tech, a woman in her mid-fifties called Anjali, poked her head through the door.

“Great. How long will it take?” Shelley asked.

Anjali twisted her mouth. “I’ve already fast-tracked one sample for you today, Shelley. We do have other cases on the roster, you know.”

“I know, but this is a local case,” Shelley said. “Your boy goes to the college, doesn’t he? Jaipinder? All the attacks so far have been on campus. The quicker we get this case wrapped up, the better.”

Anjali rolled her eyes at the obvious emotional blackmail, but nodded all the same. “I will get it through as quick as I can. No promises, though.”

“Thank you, Anjali.” Shelley smiled, offering her colleague a one-second shoulder squeeze as Dr. Applewhite joined them in the corridor.

With Anjali retreating back toward her office, Zoe turned to her mentor and gave her a nod of solidarity. “That is all we need for now. You can go home.”

“Wh—no, it isn’t,” Shelley interrupted, seemingly lost for words for a moment. “We still have a lot of questions. About the equations, for example. And we can’t just let a suspect go home without due diligence.”

Dr. Applewhite’s eyebrows shot up an inch at the use of the word “suspect.”

“That will not be necessary,” Zoe said, turning to face Shelley head-on. “I am vouching for her. She will not flee the country or go on a murder spree. We can call and let her know when the results have cleared her.”

“Zoe,” Shelley said, then caught herself and lowered her tone. She pulled Zoe’s sleeve to angle them both away from Applewhite, facing down the corridor where they could discuss more discreetly. “That’s against protocol. I know you have history, but that doesn’t matter. We do this by the book. If you get caught giving preferential treatment, we’ll be off the case at the very least.”

“She did not do anything wrong,” Zoe insisted. There was a stubborn streak in her a mile long, and Shelley had yet to come up against that. She was in for a surprise if she wanted to test it.

It was Zoe and Dr. Applewhite against the world now, and she wasn’t going to let her down. Not when Dr. Applewhite had been the only person who always had her back. She was going to fight on her behalf, and she couldn’t stand hearing the accusation and the suspicion.

“Even if she didn’t,” Shelley said, pausing with an emphasis that seemed to suggest that she was not convinced, “we still need to keep her here. Tick the boxes. This is the way we work cases, Z, and you know that. We don’t get to break the procedure just because we know someone personally.”

Zoe opened her mouth to reply, but she never had a chance.

“If I may interrupt,” Dr. Applewhite said, her tone mild. “I don’t mind staying until this is sorted out. Really, it’s no problem. I’ve already cancelled my appointments for the rest of the day, so I have nothing to rush back to.”

“But,” Zoe began, about to protest on her behalf.

“It’s really fine,” Dr. Applewhite said, firmly and with a meaningful glance her way. “I mean it. I have nothing to hide, so what’s the harm?”

Zoe’s shoulders slumped, and she couldn’t quite bear to face Shelley as she nodded assent.

The three of them marched silently back through the corridors of the J. Edgar Hoover building, out of the labs and back toward the holding rooms, to a place where they could leave Dr. Applewhite for a few hours. They took the turns and chose the right floor in the lift without discussion. Zoe did not feel up to interrogating Dr. Applewhite about the equations, and she couldn’t imagine that Shelley wanted to at that moment either.

Instead she counted their steps, listening to the rhythm and cadence of a pair of heels and two pairs of flats. The harder, heavier thud of her own boots, the slightly faster patter of Shelley’s dress shoes, her stride shorter than that of the other two women. The pattern that echoed against the walls as they fell more or less into step with one another, as humans who walk together are wont to do.

Zoe stayed out in the hall when Shelley showed Dr. Applewhite into the questioning room where she would wait for them, and asked her about wanting a drink, and made sure that she was seated comfortably. She stared straight ahead down toward the next bend, and hated herself for flinching when Shelley closed the door and locked it.

“I know you aren’t happy with me right now,” Shelley sighed. “But it’s only for a few hours. Like you said, she’s innocent. Once we have this done, we can move on to other things. Maybe someone’s targeting Dr. Applewhite by pointing to her equations. Who knows? Maybe they were there as a clue, and we just saved her life by keeping her in a secure building while the killer waits outside her apartment.”

That was some consolation, but it did put a shiver down Zoe’s spine. “You think we should assign her a police escort when she leaves? Make sure that no one is stalking her?”

“It’s worth thinking about.” Shelley cocked her head and smiled at Zoe in a way she didn’t totally understand. “You know, there’s one nice thing come out of all this. I feel like I’m getting to know you better. I didn’t know you had someone you felt so strongly about.”

Zoe was taken aback by the observation. She looked toward the door, even as she knew that there was no way Dr. Applewhite could hear them through the reinforced material. “I… I suppose we are close. Dr. Applewhite was the first person to… diagnose me. She supported me.”

“I know it can’t be easy seeing her in here.” Shelley sighed and gestured to the next door along the hall. “Come on. We can sit on the observation side and wait for the call. Keep her company, of a sorts.”

***

After several hours of continued staring at the equations, Zoe was still no closer to figuring it all out than she had been the first moment they were handed the case. No matter how she looked at them, she couldn’t figure out how they worked or even why they were broken. And worse: the more she looked, the less convinced she was that it really was a coincidence. Those last lines made a perfect copy of Dr. Applewhite’s theory.

That kind of thing didn’t happen by accident.

Shelley’s cell rang, and the two of them snapped to attention. They looked at it for a second, buzzing on the ledge in front of them, before Shelley grabbed it and answered.

“Hello, Anjali? Yes… Right. And you’re absolutely sure? Okay, thank you. Yes, I do owe you one. Well, all right, two. Thanks again.”

Shelley finished the call and put her cell down, biting her lip. She hadn’t taken her eyes off it yet, or looked up any higher than Zoe’s knee since she had answered it.

Zoe, who had observed that Shelley spent around seventy-five percent of her time looking at people’s faces, and perhaps thirty percent looking someone directly in the eye, considered this to be a very bad sign indeed.

Shelley’s face was pale when she did look up, and then she had to glance away again before she spoke. “The DNA is a match.”

Zoe waited for a moment for the punchline or an explanation. When Shelley didn’t say anything else, she had to follow up with a prompt. “A match for what?”

“For Dr. Applewhite. The hairs are hers.”

There was no response in Zoe’s head. Only silence. She sat there looking at Shelley, the words ringing hollow in the room around them, nothing but utter disbelief bouncing back.