There's a kind of mathematics that goes along with saving people's lives. You find yourself running the figures without even realizing it, like a medic on a battlefield. This patient has no chance of surviving. That one does, but only if you let a third die.

For me, the equation broke down into fairly simple elements. The demon, hungry for its revenge, would come after those who had struck it down. The ghost would only remember those who had been there, whom it had focused on in those last moments. That meant that Murphy and Michael would be its remaining targets. Michael had a chance of protecting himself against the thing - hell, maybe a better chance than me. Murphy didn't.

I got on the phone to Murphy's place. No answer. I called the office, and she answered with a fatigue-blurred, "Murphy."

"Murph," I said. "Look, I need you to trust me on this one. I'm coming down there and I'll be there in about twenty minutes. You could be in danger. Stay where you are and stay awake until I get to you."

"Harry?" Murphy asked. I could hear her starting to scowl. "You telling me you're going to be late?"

"Late? No, dammit. Look, just do what I said, all right?"

"I do not appreciate this crap, Dresden," Murphy growled. "I haven't slept in two days. You told me you'd be here in ten minutes, and I told you I'd wait."

"Twenty. I said twenty minutes, Murph."

I could feel her glare over the phone. "Don't be an asshole, Harry. That's not what you said five minutes ago. If this is some kind of joke, I am not amused."

I blinked, and a cold feeling settled into my gut, into the hollow place the Nightmare had torn out of me. The phone line snapped, crackled, and popped, and I struggled to calm down before the connection went out. "Wait, Murphy. Are you saying you talked to me five minutes ago?"

"I am about two seconds short of killing the next thing that pisses me off, Harry. And everything keeping me out of bed is pissing me off. Don't get added to the list." She hung up on me.

"Dammit!" I yelled. I hung up the phone and dialed Murphy's number again, but only got a busy signal.

Something had talked to Murphy and convinced her she was talking to me. The list of things that could put on someone else's face was awfully long, but the probabilities were limited: either another supernatural beastie had wandered onto the stage or, I gulped, the Nightmare had taken a big enough bite of me that it could put on a convincing charade.

Ghosts could take material form, after all - if they had the power to form a new shape out of material from the Nevernever, and if they were familiar enough with the shape. The Nightmare had eaten a bunch of my magic. It had the power it needed. And it had the familiarity it needed.

Hell's bells, it was pretending to be me.

I hung up the phone and tore around the house frantically, collecting car keys and putting together an improvised exorcism kit from stuff in my kitchen: Salt, a wooden spoon, a table knife, a couple of storm candles and matches, and a coffee cup. I stuffed them all into an old Scooby-Doo lunch box, then, as an afterthought, reached into a bag of sand that I keep in the kitchen closet for Mister's litter box, and tossed a handful into a plastic bag. I added the scorched staff and blasting rod to the accumulating pile of junk in my arms. Then I ran for the door.

I hesitated, though. Then went to the phone and dialed Michael's number, fingers dancing over the rotary. It was also busy. I let out a shriek of purest frustration, slammed down the phone, and ran out the door to the Blue Beetle.

It was late. Traffic could have been worse. I got there in less than the twenty minutes I'd promised Murphy and parked the car in one of the visitor's parking spaces.

The district station Murphy worked in crouched down amongst taller buildings that surrounded it, solid and square and a bit battered, like a tough old sergeant amongst a forest of tall, young recruits. I ran up the stairs, taking my blasting rod with me, with my Scooby-Doo lunch box in my right hand.

The grizzled old sergeant behind the desk blinked at me as I came panting through the doorway. "Dresden?"

"Hi," I panted. "Which way did I go?"

He blinked. "What?"

"Did I come through here a minute ago?"

His thick, grey moustache twitched in nervous little motions. He took a look at his clipboard. "Yeah. You went up to see Lieutenant Murphy just a minute ago."

"Great," I said. "I need to see her again. Buzz me through?"

He peered at me, a little closer, then reached forward to buzz me through. "What's going on here, Mr. Dresden?"

"Believe me," I said, "As soon as I work that out, I'll be sure to tell you." I opened the door and headed through, up the stairs, and toward the S.I. offices on the fourth floor. I pounded through the doors and sprinted down the rows of desks toward Murphy's office. Stallings and Rudolph both started up from their chairs, blinking as I went past.

"What the hell?" Rudy blurted, his eyes widening.

"Where's Murphy?" I shouted.

"In her office," Stallings stammered, "with you."

Murphy's office stood at the back of the room, with cheap walls and a cheap door that finally bore a genuine metallic nameplate with her name and title on it. I leaned back and drove my heel at the doorknob. The cheap door splintered, but I had to kick it again to send it swinging open.

Murphy sat at her desk, still wearing the clothes I'd last seen her in. She'd taken her hat off, and her short blonde hair was mussed. The circles beneath her eyes were almost as dark as bruises. She sat perfectly still, staring forward with her blue eyes set in an expression of horror.

I stood behind her, all in black - the same outfit I'd worn the night we'd stopped Kravos and his demon. The Nightmare looked like me. Its hands rested on either side of her face, fingertips on her temples - except that they had, somehow, pressed into her head, reaching down through skin and bone as though gently massaging her brain. The Nightmare was smiling, leaning down a bit toward her, head canted as though listening to music. I didn't know my face was capable of making an expression like that - serene and malicious and frightening.

I stared for a second in sheer horror at the weirdness of the sight. Then blurted, "Get the hell off of her!"

The Nightmare's dark eyes snapped up, sparkling with a cold, calm intelligence. It lifted its lips away from its teeth in an abrupt snarl. "Be thou silent, wizard," it murmured, steel and razor blades in its words. "Else I will tear thee apart, as I already have this night."

A little gibbering shriek of terror started somewhere down in my quivering belly, but I refused to give it a voice. I heard Rudy and Stallings coming behind me. I lifted the blasting rod and leveled it at the Nightmare's head. "I said to get off of her."

The Nightmare's mouth twisted into a smile. It lifted its hands away from Murphy, fingers just sliding out of her skin as though from water, and showed me its palms. "There is something thou hast forgotten, wizard."

"Yeah?" I asked. "What's that?"

"I have partaken of thee. I am what thou art," the Nightmare whispered. He flicked his wrists toward me. "Ventas servitas."

Wind roared up in a sudden fury and hurled me from my feet, back into the air. I collided with Rudolph and Stallings as they ran forward. We all went down in a heap upon the ground.

I lay there stunned for a moment. I heard the Nightmare walk out. It just walked past us, footsteps calm and quiet, and left the room. We gathered ourselves together slowly, sitting up.

"What the hell?" Rudolph said.

My head hurt, in back. I must have slammed it into something. I pressed a hand against my skull, and groaned. "Oh, stars," I muttered. "I should have known better than to give him a straight line like that."

Stallings had blood running out his nose and into his greying moustache. Flecks of red spotted his white dress shirt. "That ... Good Lord, Dresden. What was that thing?"

I pushed myself to my feet. Everything wobbled for a moment. My whole body shook, and I felt like I might just fall over and start crying like a baby. It had used my magic. It had stolen my face and my magic and used them both to hurt people. It made me want to start screaming, to tear something apart with my bare hands.

Instead, I staggered toward Murphy's office. "It's what got Malone," I told Stallings. "It's kind of complicated."

Murphy still sat in her chair, her eyes wide and staring and horrified, her hands folded into her lap. "Murph?" I asked. "Karrin? Can you hear me?"

She didn't move. But her breath came out with a little edge to it, as though she had tried to speak. She breathed. Thank God. I knelt down and took her hands in mine. They felt ice cold.

"Murph," I whispered. I waved my hand in front of her eyes, and snapped my fingers sharply. She didn't so much as blink.

Rudolph's handsome face was pale. "I'll call downstairs. Tell them not to let him out." I heard him go to the nearest phone and start calling down to the desk. I didn't bother to tell him that it wouldn't do any good. The Nightmare could walk out through the walls if it needed to.

Stallings joined me in the room, looking shaken and a little grey. He stared at Murphy for a long moment, and then asked, "What is it? What's wrong with her?"

I peered at her eyes. They were dilated wide. I braced myself, and looked deeper into her eyes. When a wizard looks into your eyes, you cannot hide from him. He can see deep down into you, see the truest parts of your character, the dark places and the light - and you see him in return. Eyes are the windows to the soul. I searched for Murphy behind all of that terror, and waited for the soulgaze to begin.

Nothing happened.

Murphy just sat there, staring ahead. Another low breath rattled out, not quite making a sound - but I recognized the effort she was making for what it was.

Murphy was screaming.

I had no idea what she was seeing, what horrors the Nightmare had set before her eyes. What it had taken from her. I touched her throat with gentle fingertips, but I couldn't feel the bone-chilling cold of the torment-spell like the one upon Malone. At least there was that much. But if I couldn't see inside of her, then Murphy was in another place. The lights were on, but no one was home.

"She's ... This thing has messed with her head. I think it's making her see things. Things that aren't here. I don't think she knows where she is, and she can't seem to move."

"Christ preserve," Stallings whispered. "What can we do?"

"John," I said, quietly. "I need you to pull the evidence files from the Kravos case. I need that big leather book that we found at his apartment."

Stallings started, and then stared at me. "You need what?"

I repeated my request.

He closed his eyes. "Jesus, Dresden. I don't know. I don't know if I could get it. There's been some stuff come up lately."

"I need that book," I said. "The thing that's doing this is a kind of demon. Kravos will have that demon's name written down in his spell book. If I can get that name, I can catch this thing and stop it. I can make it tell me how to help Murph."

"You don't understand. It isn't going to be that easy for me. This has gotten complicated, and I'm not going to be able to just walk into storage and get the damn thing for you, Dresden." He studied Murphy with worried eyes. "It could cost me my job."

I set my Scooby-Doo lunch box on the floor and opened it up. "Listen to me," I said. "I'm going to try to help Murphy. I need someone to stay with her until dawn and then to take her back to her house - or better yet, to Malone's house."

"Why?" Stallings asked. "What are you doing?"

"I think this thing is making her live through some messed up stuff - like in a nightmare. I'm pretty sure I can stop it, but she'll still be vulnerable. So I'm going to set up a protection around her so that she'll be safe until dawn." Once morning rolled around, the Nightmare would be trapped in whatever mortal body it possessed, or else would have to flee to the Nevernever. "Someone will need to watch her, in case she wakes up."

"Rudolph can do it," Stallings said, and rose to his feet. "I'll talk to him."

I looked up at him. "I need that book, John."

He frowned, studying the ground in front of me. "Are we going to be able to catch this thing, Dresden?" We meaning the police. I could hear that much in his voice.

I shook my head.

"If I get the book for you," he said, "can you help the lieutenant?"

I nodded at him.

He closed his eyes and let out a breath. "All right," he whispered. Then he walked out. I heard him talking to Rudolph a moment later.

I turned back to Murphy, taking the plastic sack of sand out of the lunch box. I got out a piece of chalk and pushed Murphy's chair back from the desk so that I could draw a circle around us both and will it closed. It took more effort than it usually did, leaving me dizzy for a second.

I swallowed and began to gather up energy, to focus it as carefully and precisely as I could. It built slowly, while Murphy continued to inhale, and to exhale whispered screams. I put my hand on her cold fingers, and thought about all the stuff we'd been through, the bond of friendship that had grown between us. Good times and bad, Murph's heart had always been in the right place. She didn't deserve this kind of torment.

A great fury began to stir in me - not some vaporous, swiftly dissipated flash of anger, but something deeper, darker, more calm and more dangerous. Rage. Rage that this sort of thing should happen to someone as selfless and caring as Murphy. Rage that the creature had used my power, my face, to trick its way close to her and to hurt her.

From that rage came the power I needed. I gathered it up carefully and shaped it with my thoughts into the softest-edged spell I could conceive. Gently, I sent the power coursing down my arm into the grains of sand I pinched between a single fingertip. Then I slowly lifted my arm, the spell holding in a precarious balance as I sprinkled a bit of sand over each of her eyes. "Dormius, dorme," I whispered. "Murphy, dormius."

Power coursed out of me, flowed down my arm like water. I felt it fall with the grains of sand. Murphy let out a long, shivering breath, and her staring eyes began to flutter closed. Her expression slackened, from horror to deep, silent sleep, and she slumped into her chair.

I let out my breath as the spell took hold, and bowed my head, trembling. I reached out and stroked my hand over Murphy's hair. Then I composed her into something that looked a little more comfortable. "Down where there's no dreams," I whispered to her. "Just rest, Murph. I'll nail this thing for you."

With an effort of will, I smudged the circle and broke it. Then I stepped outside it, used the chalk to close it again and willed it closed around Murphy. I had to strain, this time, more than I'd ever needed to since I was barely more than a child. But the circle closed around her, sealing her in. A small haze, only an inch or two high, danced around the chalk lines, like heat waves rising from summer roads. The circle would keep out anything from the Nevernever - and the enchanted sleep would hold until the dawn came, keep her from dreaming and giving the Nightmare a way to further harm her.

I shambled out of her office, to the nearest phone. Rudolph watched me. Stallings wasn't in evidence. I dialed Michael's number. It was still busy.

I wanted to crawl home and drop a sleep spell on myself. I wanted to hide somewhere warm and quiet and get some rest. But the Nightmare was still out there. It was still after its vengeance, after Michael. I had to get to it, find it, stop it. Or at least warn him.

I put the phone down and started gathering my stuff together. Someone touched my shoulder. I looked up at Rudolph. He looked uncertain, pale.

"You'd better not be a fake, Dresden," he said, quietly. "I'm not really sure what's going on here. But so help me God, if something happens to the lieutenant because of you ..."

I studied his face numbly. And then nodded. "I'll call back for Stallings. I need that book."

Rudolph's expression was serious, earnest. He'd never much liked me, anyway. "I mean it, Dresden. If you let Murphy get hurt, I'll kill you."

"Kid, if anything happens to Murphy because of me ..." I sighed. "I think I'll let you."