Her mouth and eyebrows quirked in an ironic smile and she passed in front of him. His hand flashed in and out of his jacket pocket, emerging with the heavy cosh. She must have registered some movement, for she half-turned as his arm descended in a swift arc towards her head. He had meant to hit her firmly on the back of the head, but caught her on the temple. She staggered and moaned, but didn’t go down. Instead, she stumbled towards him. Panicked, he raised the sap again and smashed it down on the crown of her head. This time, she crumpled in an awkward heap at his feet. He gasped in relief, his head swimming. After what had happened with Schilling, even the slightest glitch was enough to provoke the momentary clutch of terror in his chest. But it was fine, he told himself. Everything was fine.

He crossed to the door and flipped the catch, locking them in. Then he hurried to the desk and swept all the books and papers to the floor in an untidy heap. He turned to Dr Calvet and bent to pick her up. She was light as a child in his arms, which was a welcome change from his first three victims. He laid her on her back on the desk and took the cords from his holdall. It was the work of moments to fasten her wrists and ankles tightly to the metal feet. He flicked up an eyelid with his thumb. She was still out cold. No need to gag her. He was back in control.

He took his grandfather’s cut-throat razor from its case and painstakingly cut her clothes away. There was scarcely a scrap of flesh on her bones. If he’d felt inclined, he could have run his fingers over her ribs like the beads on an abacus. He stepped back for a moment, savouring her exposed defencelessness.

Suddenly he felt desire well up inside him, a richness in his blood that made him almost dizzy. Until now, he’d always refused to acknowledge that the surge of adrenaline-fuelled urgency that swept through him when confronted with his victims had anything to do with sex. There was no place for carnal desire here. Sex was for afterwards.

But perhaps he’d been wrong. He took a deep breath, noticing the citrus tang of her perfume overlaying the more human scent of her naked flesh. Why settle for low-life whores when he could take what he wanted from his victims? Didn’t they deserve that final humiliation, to be fucked over like they’d fucked over their own victims?