“Let’s do the couch,” Rizzoli cut in.

Mick redirected the lens. The couch fabric also glowed under UV, but it was a softer fluorescence, like snow under moonlight. Slowly he scanned the padded frame, then the cushions, but spotted no suspicious smears, only a few long stray hairs and dust particles.

“These were tidy people,” said Mick. “No stains, not even much dust. I’ll bet this couch is brand-new.”

Korsak grunted. “Must be nice. Last new couch I bought was when I got married.”

“Okay, there’s some more floor space back there. Let’s move that way.”

Rizzoli felt Korsak bump into her, and she smelled his doughy odor of sweat. His breathing was noisy, as though he had sinus problems, and the darkness seemed to amplify his snuffling. Annoyed, she stepped away from him and slammed her shin against the coffee table.

“Shit.”

“Hey, watch where you’re going,” said Korsak.

She bit back a retort; things were already tense enough in this room. She bent down to rub her leg. The darkness and the abrupt change in position made her dizzy and disoriented. She had to squat down so she wouldn’t lose her balance. For a few seconds she crouched in the blackness, hoping Korsak wouldn’t trip over her, since he was heavy enough to squash her flat. She could hear the two men moving about a few feet away.

“The cord’s tangled,” said Mick. The Crimescope light suddenly shifted in Rizzoli’s direction as he turned to free up the power cord.

The beam washed across the rug where Rizzoli was crouched. She stared. Framed by the background fluorescence of the rug fibers was a dark irregular spot, smaller than a dime.

“Mick,” she said.

“Can you lift that end of the coffee table? I think the cord’s wrapped around the leg.”

“Mick.”

“What?”

“Bring the scope over here. Focus on the rug. Right where I am.”

Mick came toward her. Korsak did, too; she could hear his adenoidal breathing moving closer.

“Aim at my hand,” she said. “I’ve got my finger near the spot.”

Bluish light suddenly bathed the rug, and her hand was a black silhouette against the fluorescing background.

“There,” she said. “What is it?”

Mick crouched beside her. “A stain of some kind. I should get a photo of that.”

“But it’s a dark spot,” said Korsak. “I thought we were looking for something that fluoresces.”

“When the background’s highly fluorescent, like these carpet fibers, body fluids may actually look dark, because they don’t fluoresce as brightly. This stain could be anything. The lab will have to confirm it.”

“So what, are we gonna cut a piece out of this nice rug, just because we’ve found an old coffee stain or something?”

Mick paused. “There’s one more trick we can try.”

“What?”

“I’m going to change the wavelength on this scope. Bring it down to UV shortwave.”

“What does that do?”

“It’s real cool if it happens.”

Mick adjusted the settings, then focused the light on the area of rug containing the dark blot. “Watch,” he said, and flipped off the power to the Crimescope.

The room went pitch-black. All except for one bright spot glowing at their feet.

“What the hell is that?” said Korsak.

Rizzoli felt as though she were hallucinating. She stared at the ghostly image, which seemed to burn with green fire. Even as she watched, the spectral glow began to fade. Seconds later, they were in complete blackness.

“Phosphorescence,” said Mick. “It’s delayed fluorescence. It happens when UV light excites electrons in certain substances. The electrons take a little extra time to return to their baseline energy state. As they do, they release photons of light. That’s what we were seeing. We’ve got a stain here that phosphoresces bright green after exposure to short-wavelength UV light. That’s very suggestive.” He rose and switched on the room lights.

In the sudden brightness, the rug they had been staring at with such fascination appeared utterly ordinary. But she could not look at it now without feeling a sense of revulsion, because she knew what had taken place there; the evidence of Gail Yeager’s ordeal still clung to those beige fibers.

“It’s semen,” she said.

“It very well could be,” said Mick as he set up the camera tripod and attached the Kodak Wratten filter for UV photography. “After I get a shot of this, we’ll cut out this section of the rug. The lab will have to confirm with acid phosphatase and microscopic.”

But Rizzoli needed no confirmation. She turned toward the blood-spattered wall. She remembered the position of Dr. Yeager’s body, and she remembered the teacup that had fallen from his lap and shattered on the wood floor. The spot of phosphorescing green on the rug confirmed what she had feared. She understood what had happened, as surely as if the scene were playing out before her eyes.

You dragged them from their beds to this room, with its wood floor. Bound the doctor’s wrists and ankles and taped over his mouth so he could not cry out, could not distract you. You sat him there, against that wall, making him your mute audience of one. Richard Yeager is still alive, and fully aware of what you are about to do. But he cannot fight back. He cannot protect his wife. And to alert you to his movements, his struggles, you place a teacup and saucer on his lap, as an early-warning system. It will clatter on this hard floor should he manage to rise to his feet. In the throes of your own pleasure, you cannot keep an eye on what Dr. Yeager is doing, and you do not want to be taken by surprise.

But you want him to watch.

She stared down at the spot that had glowed bright green. Had they not moved the coffee table, had they not been searching specifically for those trace leavings, they might have missed it.

You claimed her, here on this rug. Took her in full view of her husband, who could do nothing to save her, who could not even save himself. And when it was done, when you had taken your spoils, one small drop of semen was left on these fibers, drying to an invisible film.

Was killing the husband part of the pleasure? Did the unsub pause, his hand gripping the knife, to savor the moment? Or was it merely a practical conclusion to the events that preceded it? Did he feel anything at all as he grasped Richard Yeager by the hair and pressed the blade to his throat?

The room lights went off. Mick’s camera shutter clacked again and again, capturing the dark smear, surrounded by the fluorescent glow of the rug.

And when the task is done, and Dr. Yeager sits with head bowed, his blood dripping on the wall behind him, you perform a ritual borrowed from another killer’s bag of tricks. You fold Mrs. Yeager’s spattered nightgown and place it on display in the bedroom, just as Warren Hoyt used to do.

But you are not finished yet. This was just the first act. More pleasures, terrible pleasures, lie ahead.

For that, you take the woman.

The room lights came back on, and the glare was like a stab to her eyes. She was stunned and shaking, rocked by terrors that she had not felt in months. And humiliated that these two men must surely see it in her white face, her unsteady hands. Suddenly she could not breathe.

She walked out of the room, out of the house. Stood in the front yard, drawing in desperate breaths of air. Footsteps followed her out, but she did not turn to see who it was. Only when he spoke did she know it was Korsak.

“You okay, Rizzoli?”

“I’m fine.”

“You didn’t look fine.”

“I was just feeling a little dizzy.”

“It’s a flashback to the Hoyt case, isn’t it? Seeing this, it’s gotta shake you up.”

“How would you know?”

A pause. Then, with a snort: “Yeah, you’re right. How hell would I know?” He started back to the house.

She turned and called out: “Korsak?”