THIS ENDS WHEN the case does.

The words echoed in her head as she spent the next twenty-four hours working with Dr. Gibson in the Nighthawks’ laboratory, studying vial after vial of vaccine to discover if someone at Kestrel Laboratories was behind the tampering, or whether it occurred purely at the clinics themselves.

Ava felt raw and on edge, barely able to concentrate. It wasn't like her at all, and she kept watching the clock, counting down the hours until this thankless job was done.

Kincaid had opened her eyes to passion, and she felt like her body was in control now, demanding more of the enticing drug. The need for his body warred with her desire to complete the job. Rational impulses versus primal. It was probably a good thing he wasn't here, or she'd be virtually useless.

"Well, that's the last batch," Dr. Gibson said, dusting off his hands and looking tired. "Not a single tampered sample."

Kestrel Laboratories used glycerin to both conserve the vaccine and make the virus inert, which took time and a low temperature. The vaccine vials were kept in the icebox at the clinics, so it wasn't as though someone could have injected a live specimen of the craving virus into the vaccine. The glycerin would have acted to damage the new viral cells. So the saboteur had to have replaced the vial, leaving out the glycerin or any antiviral agent. Ava capped the last vial she'd been checking, and placed it in the tray to be destroyed with the rest of them in the Nighthawks’ incinerator. "Kestrel's innocent. All of their samples are perfect, and they claim the vaccine is transported under security from its facility to the clinics, following the picketing against the clinics that happened when they initially opened. So they don't think it happened on their end."

"So it's the clinics, lass," Gibson said, casting his apron aside. "Hell of a case."

Considering the bomb planted in the drones, she'd already concluded a break-in had occurred, but they had to be certain. "Thank you for your help. Let's hope it was only the one batch of vaccines that were sabotaged."

Gibson flicked through the roster of patient names one of the Nighthawks had collected from Dr. Harricks. "There were one hundred vials of vaccine ordered in the batch that contained Mr. Thomas's afflicted sample. Marcus Long was also one of the patients seen that week at the clinic, so that makes at least two out of a hundred who were stricken with the craving."

"Dr. Harrick's statement claimed the batch arrived on the nineteenth and he didn't unpack it until the clinic was closed, so it could only have been touched that night—or one of the following nights. He ordered a new batch on the twenty-third, as he was running low. If we presume that batch was the only one tampered with, then we have a four-day window during which someone broke in and sabotaged the vaccine."

"Both David Thomas and Marcus Long were seen on the twenty-first," Gibson pointed out.

"So a two-day window, and potentially a hundred patients who might have been stricken, if it happened the first night."

"I'll mention it to Garrett and he can see all of the patients are questioned."

Thank God. It was a monumental task, let alone the fact Francis Jenkins, the other vaccinated victim, had been a patient of the clinic on Church Street, in Marylebone, which significantly widened the victim pool. Ava rubbed her temples. "I have a headache just thinking about it."

"Then don't. Go home, get some rest. I'll handle this end of the investigation and let you know the results." Gibson leaned against the steel workbench. "I don't entirely know what this is all about," he said, "as Garrett's keeping the investigation into the bombing quiet, but you're not getting in over your head, are you? With this mysterious employer you've left me for?"

Ava patted his arm warmly. "You worry too much, Doctor. And my employer's offer was better than yours." A means to spread her wings outside this laboratory, and perhaps work a case of her own, one that might save London.

Gibson clamped a hand over his heart. "Aye, lass, you've a wounding tone. What could be better than working here with me?"

Ava's smile died. This room held its own ghosts for her. Once, she'd thought it a safe haven, but lately she'd begun to wonder if it was becoming a cage. "I need something more."

"I know you do," he said, kissing her on the forehead in a grandfatherly fashion. "Just don't get yourself killed while you work out what that something more is."

"It's not as though I lead an exciting life, Doctor," she scoffed, heading for the door and her coat and scarf. "I'm a laboratory assistant, and a crime scene investigator. What on earth could hurt me?"

"Someone clearly didn't want it to be known the vaccine had been tampered with. And we don't know whom. Or why they did it."

She looped her scarf around her throat. "'Yet,' as you always say," she said, "Someone, somewhere has slipped up. I just have to work out how and where and when. Everybody leaves a trace, or a secret, or a witness. Leave no stone unturned, and whatnot."

Gibson couldn't help rolling his eyes. "Get out of here before it's too late. You're even starting to sound like me."

Ava smiled to herself as she exited the room. "Not an entirely bad thing, Doctor. You're efficient, if nothing else."

* * *

Something bothered Ava about the case.

Oh, not about the vaccine. That trail led to a dead end for the moment, but she couldn't help picturing Mr. Thomas's black-veined face.

If she put the facts together she could fill in enough gaps: Mr. Thomas, a staunch humanist, received his vaccine six weeks ago, not knowing the vial was tampered with.

He began to exhibit signs of the craving virus, though they'd likely have been minimal and he might not have even known until it was too late.

And then something killed him.

It wasn't the vaccine. But was it the virus someone had changed the vaccine with? Had he been infected with some sort of mutated craving virus? She'd never heard of any complications, but then... that didn't mean there were none.

Ava frowned, pacing the small laboratory she'd set up in Malloryn's safe house. "No," she whispered to herself, thinking about Zero, the dhampir woman who'd been found dead in her basement cell in Malloryn's hidden safe house, black veins streaking like obsidian lightning through her skin. There'd been an injection site on her body; evidence someone injected something into her, which killed her.

But what?

Poison? Hemlock was the only thing that had been discovered to have an effect on a blue blood, and that wore off in minutes, depending on how high the blue blood's CV levels were.

It couldn't be some rare mutated form of the craving virus, one that killed its host as it tried to transform them. Because it wasn't isolated solely to blue bloods.

True, dhampir were evolved from blue bloods, a step along the evolutionary chain, if one had read The Origin of Species, as she had. They required an elixir vitae to help with their ultimate transformation, but their blood work was just different enough to a blue blood's, and their bodies even more invulnerable to harm.

So she now knew how Mr. Thomas became a blue blood. She just didn't know what had killed him. Or Zero. Or the other four victims.

She felt like she was missing something... like a thought hovering at the edge of her mind, but the more she chased after it, the more it dissolved into nothing.

"Penny for your thoughts," said a deep voice from the doorway.

Ava spun with a gasp, all of her senses heightening when she saw Kincaid resting a shoulder against the doorway. She'd barely seen him since last night, after they parted ways when they returned to Malloryn's safe house—she to the guild, and he... to do whatever it was that kept him busy today.

"Sorry," he said, looking anything but apologetic. "I thought you'd have heard me, or smelled my cologne."

Little more than twenty-four hours ago, he'd pinned her to the wall of the art gallery and driven her to the point of orgasm. And she was clearly not the only one reminiscing, judging by the twinkle in his blue eyes.

When he smiled like that he stole her breath. The slightly crooked slant of his nose, the fullness of his mouth, and the faint dimple on the right side of his mouth stirred her in ways she couldn't quite comprehend. She could still taste that mouth on hers, and Ava swiftly looked away, trying to busy her hands before he noticed her fascination with him. They didn't have time to play games today, but he'd promised her the next time he kissed her, he didn't intend on stopping.

"Busy day?" she asked.

Kincaid stepped inside the room, taking it in. "I slept," he admitted, "since you had little use for me, and then I visited Orla and Ian this morning, and returned for lunch. I'm not much help with the laboratory work, I'm afraid."

She filled him in on what she and Dr. Gibson had found.

"Something is vexing me," she admitted, pushing thoughts of Kincaid's mouth and body out of her mind. "If someone killed Mr. Thomas, then that someone knew he was a blue blood. How? Was it someone watching the clinics? The same person who tampered with the vaccine and set the bombs? Did they tamper with all the vaccine vials and track every single victim down? The lack of bodies on the ground suggests otherwise, as there were at least ninety-eight other patients through the clinic during the time period we've nailed down, and there's been no outcry. We'd have noticed if people were finding more bodies. So why Mr. Thomas? Why Marcus Long? Why Francis Jenkins, John Redmond, or Quentin Longbow? What made these gentlemen stand out as men to die?"

"Can't help you, I'm afraid."

She sighed and lowered her head, resting her hands on one of her benches. There were no answers to that question, not yet. But if she kept asking questions, then maybe she'd jog loose whatever thought kept teasing her. "I'm missing something. I'm sure of it. There's something about this case I feel I should know."

His hands settled on her shoulders. "You've done an amazing job already. I'm in awe of your thought processes."

Awe? She swallowed a little, still feeling the weight of a thousand other rejections over the years. "It's nothing, really. Dr. Gibson helped me figure out most of it."

Kincaid turned her around, his black brows drawing together. "Ava, I've been at your side for most of this case. Dr. Gibson might be an accessory to the thought process, but you're the one in charge. You're the one who's putting this altogether." He arched a brow. "I feel fairly bloody useless, to be honest."

"You're not useless," she protested. "You saved my life at the clinic when the bomb detonated. I wouldn't be here—"

"It wouldn't have detonated if I hadn't opened the back panel."

"Yes, but then whoever set it could have triggered the detonation at any time they wished, and we'd know no better." She glared up at him. "I wouldn't have even made it to Mr. Thomas's house in the first place without getting caught in that riot."

"So I'm to provide some muscle, am I?"

"I'm sure you'll come in handy," she replied, not quite looking at the breadth of his chest.

"Aye, when someone needs a bunch of fives," he snorted.

"When I need you," she said quietly, and the night he'd put his coat around her shoulders at the Garden of Eden sprang to mind. "I couldn't do this without you. I grow hysterical sometimes, when I cannot even help it. Here, looking at vaccines, and evidence, and bodies, I'm in my element. It all makes sense to me, and I'm in control. Out there"—she gestured to the windows—"I'm fighting to keep my equilibrium. You know London like the back of your hand. You know its people and the way they think. I'm merely a bystander, plucking clues from what they leave behind. You're more important than you think."

The intensity of his gaze burned her. "You're my anchor," she whispered, "my link to a world I sometimes don't understand."

Kincaid twirled something in his fingers; a flash of color, quickly contained. "I'm not going to keep arguing over which one of us is more useful than the other. You win. You couldn't do without me."

A laugh escaped her, but he reached out, brushing the curl of hair that had escaped her chignon back behind her ear, and when he removed his hand, there was something else tucked there.

Ava tugged it free, catching a hint of its dark, sultry scent. Brilliant magenta petals draped lushly over her palm. "Cattleya labiata," she breathed. "Oh, my goodness, where did you get this?" Then horror dawned. "You cut the flower off the orchid?"

"I told you I was busy today. Do you like it?"

"Yes!" Even if he'd beheaded it. Ava cupped the precious bloom in her palms. "This was the first orchid species Mr. William Swainson sent back from Brazil in 1818. I've seen it in books, but never...." Never in person.

She felt almost breathless. He'd picked a flower for her. No, he'd gone out of his way to find an orchid, because he knew she adored them, and had a fascination with plants. Ava looked up, a smile spreading across her face, and that's when the tickling knot at the back of her mind suddenly unraveled, almost as if she'd pulled a thread. Suddenly her mind made one of those leaps of intuition it sometimes did, and Ava's gaze drifted past Kincaid and settled on the bookshelf.

A bookshelf groaning beneath the weight of over a dozen books on rare plants from various parts of the worlds, some with healing properties, some poisonous....

"That's it," she whispered, and hurried toward the bookshelf, still cupping the orchid. Excitement bloomed. Where the devil was it?

"What's it?" Kincaid followed her, but Ava paid him no mind.

She let her fingers run over the leather-bound spines of the books, pulling them out and then discarding them on her desk until she had a pile. She was going to crush the orchid, so she tucked it back behind her ear, flipping through a book.

"I kept thinking it's not a disease that killed Mr. Thomas. There's no viral or bacterial interference, and no other agent can cause such destruction within a blue blood's body. It has to be a toxin or a poison, and I remember... there was something I read once...."

A poison, herb, or toxin that affected blue bloods and their evolved cousins the dhampir in a different way than it did humans?

Maybe her interest in obscure plants could be the make-or-break lead this case needed? Ava tossed the book aside, and pulled down another. "Rare Plants from the Himalayas." Kincaid picked up a book and read the spine as she flicked through the pages of hers, before his words penetrated.

Ava stole the book from his grasp and turned to her reading desk in the window, rifling through the careworn pages. Himalayas... that rang a bell. "We're searching for a toxin," she told him, "one that strikes down blue bloods."

"Hemlock—"

"Paralyzes them. It does something to a blue blood's blood pressure and their muscles. I've looked into it, but I'm not quite certain what it does on a cellular level. Unfortunately, there aren't many blue bloods that will allow me to paralyze them momentarily while I take samples. And I clearly cannot use myself as a test subject. I tried once, but by the time the paralysis wore off my blood cells had returned to normal, though my CV levels were exacerbated. No,"—she traced her finger over the page, ticking off plant names mentally—"hemlock has nothing to do with this. But I do recall something.... A plant that came from the Himalaya region I was warned never to use against a blue blood. I was researching a toxin, or a weapon, something to take down the dhampir with. It was just a throwaway line in a book I flagged as interesting, but didn't get time to pursue. Everything with Zero happened so quickly."

"A plant that could destroy one of the dhampir?"

"Maybe." Ava's finger paused on a pair of words, excitement flooding through her. "Caterpillar mushroom. That's it! It's grown above an elevation of three or four thousand feet in the Himalayas, and I always thought it an unusual plant, as the lower part is a caterpillar, and the upper part is a fungus. Basically, the fungus spores land on the caterpillar and as it grows the caterpillar dies.

"It has tonic properties, I believe. Or the Chinese certainly believe so. I read transcripts of a Tibetan medical text by Zurkhar Nyamnyi Dorje about its aphrodisiac properties. They call it Yartsa gunbu. And that," she said, snapping the book closed, "is the limit of my knowledge. Beyond, do not touch if one is a blue blood."

"Interesting."

Ava couldn't quite read the tone of that one word. "What do you mean? You keep looking at me with that strange expression on your face."

"I'm just... you're frightfully intelligent, did you know?"

Her heart thudded in her chest. "Frightfully?"

He looked at her, his eyes narrowing as if he saw right through her. "Poor choice of words. You're astoundingly intelligent."

It still made her feel a little discomforted. Her ex-fiancé, Paul, had been wary of her thought processes, until she'd learned to censor herself and not delve into such topics that interested her.

Kincaid leaned toward her, bringing his lips close to her ear, "Your great, big intellect makes me want to do naughty things to you, Miss McLaren."

This man was clearly not a small-minded man like Paul. Ava caught her breath.

He continued, "Maybe one day you can spout all of these big words at me while I run my hands beneath your skirts, and—"

"Kincaid!" she gasped, and he burst into laughter.

Ava slapped him on the arm, her face burning. She had the sudden urge to kiss him again, just to see if her memories of last night's events were quite as overwhelming as they'd seemed, or whether she'd simply been caught up in the moment of her first assisted orgasm.

"Has no one ever flirted with you before?”

Ava snapped the book shut, and set it aside. "Of course they have. I was engaged once. There was flirtation, though... decidedly more mild than your so-called attempt."

"So-called attempt? I see I'm not succeeding very well. Perhaps I should press my endeavors?" He stepped closer, backing her against the desk and trailing his fingers down over the lace that covered her breasts. "I keep thinking about these pretty tits."

Ava's breath caught. "You're so vulgar."

"You're entirely too innocent. And," his voice dropped, "you have no idea."

He'd startled her again. What was it about this man that made her enjoy his flirtation so much? She knew they were all kinds of wrong for each other, but she simply couldn't help herself.

Ava gave him a sidelong glance. "Perhaps, Mr. Kincaid, I could imagine."

Kincaid’s smile grew soft and heated as he rested his knuckles on either side of her hips. "That's the spirit. Now tell me... engaged? I didn't know that."

"It was a long time ago. Before Hague kidnapped me. Actually," she amended, "Hague's kidnapping is the reason my engagement ended." Storm clouds brewed in her heart, an old hurt she'd never quite gotten over. "By the time I returned home to see my family, they'd moved on. Paul thought I was dead, and he'd already become engaged to someone else." Only six months missing, and she'd been replaced, as easily as if she didn't matter.

"He's a fool then."

Ava's shoulders relaxed. "Why do you always know the perfect thing to say?"

"Haven't you realized yet?" he drawled. "I'm the perfect man."

"Perfectly ridiculous," she said, pushing at his chest. But she was smiling again, her woes forgotten. "Now stop distracting me. We have a case."

"And now we have a lead. So what's the next step? How do we find this caterpillar mushroom?"

Ava finally unleashed a smile. "I know just the place."