Cosmic Pharmacy was stationed at the far end of Flicker Street, marked by a neon cross on the ground floor of a brownstone building. Automatic doors rose as we approached them, and we walked straight into a little room packed with boxes of pills. The windows were so grimy that it was dark as night, and the only artificial lights to guide us were the signs above the shelves.

Five feet in front of us, Petr Fitzroy was standing behind a steel counter. As soon as his gaze landed on us, he turned and ran.

"Shit!" I vaulted over the counter and sprinted after him. "Riannon must have told him that we've got Nora!"

Which meant he was going back to their flat to destroy her work.

We burst through a store cupboard in the back and out the rear door, following Petr down an alley and back onto Flicker Street. The air was turning muggy, and sweat pooled across my skin. Still, I pushed myself harder, until I was running fast enough to stay at Alex's side.

But Petr had gained a good head start, and he reached his block before us. He dashed inside. When we entered the lobby, he was standing in the lift. The bloody thing was obviously working again because the doors slammed shut, and he went up.

"Stairs!" I said. It was all I had the breath for.

We took the stairs two at a time, thundering up like a million insane robots. As we reached the final step, my tired feet caught each other, and I pitched towards the floor. Alex grabbed me by the arm and practically dragged me into the hallway.

Petr was standing outside his front door. It had just finished scanning his face, and he glanced towards us as it swung open.

I drew my weapon. "Don't move!"

Petr slowly raised his hands in the air.

When we reached him, Alex patted him down. Petr ground his jaw, his gaze stormy. But he said nothing.

Alex stepped back, and I nodded at the doorway. "Keep your hands in the air and walk us inside."

Petr turned around and entered the flat. We followed.

"Kitchen," I said.

The three of us entered the kitchen. The island unit in the middle was still littered with paper and tablets, and the leaning tower of washing-up in the sink had hardly diminished.

I kept my gun trained on Petr. "I assume you know that we've arrested Nora for the murders."

"Yes." His voice had a cutting edge.

"Given your flight from the pharmacy, I also assume that you are aware your wife has been creating medicinal drugs illegally."

"Yes."

"Did you fake her alibi for Sunday evening? Did she kill Lonn Temple and Frankie Jarsdel?"

He ground his jaw and said nothing.

Alex had also drawn his pistol, but he hadn't raised it yet. "Where's her work?"

Petr shifted, and his gaze slid away. "Gone."

My eyes followed his to the washing up. Now that I was looking closely, I didn't think it had diminished at all. There were still wine glasses, plates, a greasy butter knife, a glass beaker, a tower of colourful mugs, and a mixing bowl.

I blinked. Let my eyes trail back up the pile. A glass beaker?

"She's been experimenting," I said. "She's put something in the beaker that she wants to wash out."

Petr swallowed hard.

"We'll only ask you one more time. Where's her work?"

He stared down the barrel of my gun, sweat beading on his forehead. Then he edged past Alex and into the hallway.

We followed him to the end, where he pushed the final door open. As it swung away from us, I hauled him back by the shoulder. "Stay out here with Sergeant Sullivan."

I entered a spare bedroom -- or, at least, what was supposed to be a spare bedroom. A good-sized window on the right had been thrown open to air it. A single bed was tucked into one corner, and there was also a wardrobe. But test tube racks were balanced on the mattress, and a microscope was sitting on top of the wardrobe. In the furthest corner of the room, there was a desk, and a blue folder was lying on it, open.

I sat in the hover chair behind the desk. This was the project Lonn had been working on. Although the equations were still gibberish to me, I'd looked at our true note enough times this morning to be able to see the similarities.

I laid down my pistol and snapped on a pair of nitrile gloves. Then I began to turn the pages as if I was handling an ancient relic, working my way back to the start of the folder.

As I turned the final page, Petr stepped over the threshold in my peripheral vision -- followed fluidly by a reprimand from Alex. "No!"

My gaze remained fixed on the messy title page. Three names had been written across the top.

"It looks like I have a confession to make," Petr said.

I raised my eyes.

The barrel of Alex's gun touched the back of Petr's head. "Don't move."

Petr stopped, his hands still raised.

I stood up. "You were an active member in this experiment, too?"

"I know I've taken some wrong turnings, but I love Nora. I don't want her to die." He swallowed. "All three of us were researchers. It was our project."

"And then Lonn threatened to take it away." I picked up my gun and rounded the desk. "It's you, isn't it? The murderer is you."

Petr's hands fell to his sides. "Yes."

"With Nora?"

"No!" His face crumpled. "Nora had nothing to do with it. She wasn't even here when Frankie died -- she just lied to protect me. She was fetching groceries from Ace's."

"Doing the weekly shop while you killed a young girl," Alex said coldly.

"It was Lonn's fault!" Petr's voice rose. "It's all because he was going to kill Nora. He was the first man to put his hands on that scalpel -- not me! He went for my wife. I had to protect her!"

"You stabbed him five times," I said. "That's an excessive form of defence."

"I don't think so. Look, Nora was experiencing side effects, and they were escalating. I suggested we should stop and go back to the drawing board, but Lonn was afraid we wouldn't be able to keep our secret for much longer. He'd rather Nora died than face a prison sentence, so he shut the whole operation down."

"And he attacked Nora?"

"She was angry. It was her life Lonn was talking about. She threatened to blow the whistle if he refused to carry on helping us -- his resources were vital, and she's struggled without them. But he didn't stand down. When she repeated her threat, he flew into a rage and picked up the scalpel."

I thought of the fight between Lonn and Paris, and I found the story wasn't too hard to imagine.

"He'd been drinking," Alex added quietly.

Petr nodded. "He swore he was going to silence us. He went at Nora. I stopped him. We fought. It's a blur until I'd got him on the floor. Then I was stabbing him and stabbing him. I just wanted to know he would stay away from my wife."

He looked down and opened his hands as if the scalpel was still in them.

"And then?" I said.

"I argued with Nora about what had happened. Eventually, we took the project and left."

I felt nauseous. "And you went on a date with Riannon like nothing had happened."

"What about Frankie?" Alex asked.

"She came to see Nora on Sunday afternoon," Petr said. "Nora was at Ace's, so I answered the door. I invited her in to wait, and she warned me that she believed my wife was a killer.

"She was friends with Nora, and she knew that the PET scan results had been bad. She knew Nora was dying. She also knew Lonn had been working on a project that had vanished. So she pieced it together and made a jump. The wrong jump. She wanted to turn Nora in."

"So you slipped Xanax in her drink," I said. "I assume it came from the pharmacy?"

Petr shrugged.

"Okay, you slipped Xanax in her drink, and once she was under the influence, you suffocated her. Then you dumped her body in the lift shaft." I frowned. "But why did you throw down fake papers?"

He kept his mouth shut.

I stepped forward so that my pistol was almost touching his head. The three of us were by the open window now, framed in the glowing summer's light. "Why the papers?"

He closed his eyes. "We'd spent the weekend writing them. We were going to hide the papers in the lab, then wait for someone to find them. Because they'd no longer be missing, you'd stop thinking they were relevant, and we'd be safer. But when Frankie died, I thought I could make it look like she'd killed Lonn to steal the notes for herself and then committed suicide." He heaved a deep sigh. "I was just trying to save Nora."

A hint of sympathy touched me as I looked at him. At the heart of this mess, a woman was dying, and a man was trying to save her. But I couldn't accept the lengths he'd gone to.

I holstered my pistol and reached for my handcuffs. "It didn't have to be this way, Petr."

He shook his head, tears welling in his eyes. "All I've ever done has been for Nora. But I know she'll be prosecuted for helping me now, and she'll die before her sentence is over." Desperation strangled his voice. "I won't be hung for this!"

He dived towards the window.

Alex caught him by the arm just before he fell out of reach. The force yanked him forward. His hand found purchase on the wall at the last moment, and he hung out of the window, struggling to hold Petr.

I leaned out with him, but I immediately met a swinging fist. I stumbled back, sparks exploding through my cheek.

As I regained my balance, Alex swayed. I steadied him with one hand. "Dizzy spell? Great timing."

I shoved my head out the window again. Petr was hanging above the quiet street like a shirt on a clothing line, red in the face. He thrashed as I tried to catch hold of him. "All I've ever done has been for Nora!"

I caught his wrist. "Then say goodbye to her before you go. Help us!"

He hung limply for a moment. But as we began to reel him back inside, he closed his hand over Alex's forearm and held on.

To this day, I don't know how we managed to heave him through the window. When he was in, we dropped him like a sack of potatoes. I knelt down and pressed my knee into his back before he could rise. Alex fell beside me, white as a sheet.

"Alex?" I secured my handcuffs around Petr's wrists. When they were locked together, I looked up. "Alex!"

He clenched his jaw, pressing one hand against his forehead. "I think I'm okay."

"Fucking contacts. You're taking them out as soon as I've said this." I looked at the man lying beneath me. "Petr Fitzroy, I am arresting you for the murders of Lonn Temple and Frankie Jarsdel. You do not have to say anything, but it may harm your defence if you do not mention, when questioned, something which you later rely on in court. Anything you do say may be given in evidence."