"The guy in the tracksuit?" Cassie said, and I heard that tautening at the edges of her voice: any minute now. "No, he told us that one. He just wasn't very convincing. We thought he was making a big deal out of nothing."

"You see what I mean? He was supposed to have sex with her, hit her on the head with a rock, and leave her body somewhere on the dig or in the wood. That was what I wanted. For God's sake, you'd think that would be simple enough even for Damien, but no. He didn't get a single one of those right. My God, he's lucky I just broke up with him. After the mess he made of this, I should have put you people on to him. He deserves whatever he gets."

And there it was: all we needed. The breath went out of me with a strange, painful little sound. Sam slumped back against the side of the van and ran his hands through his hair; O'Kelly gave a long, low whistle.

"Rosalind Frances Devlin," Cassie said, "I arrest you on suspicion that, on or around the seventeenth of August of this year, at Knocknaree in County Dublin, you did murder Katharine Bridget Devlin, contrary to common law."

"Get your hands off me," Rosalind snapped. We heard scuffling, the crunch of twigs snapping underfoot; then a swift, vicious noise like the hiss of a cat, and something between a smack and a thump, and a sharp gasp from Cassie.

"What the fuck-" said O'Kelly.

"Go," Sam said, "go," but I was already scrabbling for the door handle.

We ran, skidding around the corner, down the road towards the entrance of the estate. I have the longest legs and I outpaced Sam and O'Kelly easily. Everything seemed to be streaming past me in slow motion, swaying gates and bright-painted doors, a toddler on a tricycle gazing up open-mouthed and an old man in suspenders turning from his roses; the morning sunlight trickled down leisurely as honey, achingly bright after the dimness, and the boom of someone slamming the van door echoed on forever. Rosalind could have snatched up a sharp branch, a rock, a broken bottle; so many things can kill. I couldn't feel my feet hitting the pavement. I swung round the gatepost and threw myself up the main road, and leaves brushed my face as I turned onto the little path along the top wall, long wet grass, footprints in muddy patches. I felt as if I were dissolving, autumn breeze flowing cool and sweet between my ribs and into my veins, turning me from earth into air.

They were at the corner of the estate, where the fields met that last strip of wood, and my legs went watery with relief when I saw they were both on their feet. Cassie had Rosalind by the wrists-for an instant I remembered the strength of her hands, that day in the interview room-but Rosalind was fighting, intently and viciously, not to get away but to get at her. She was kicking at her shins and trying to claw her, and I saw her head jerk as she spat in Cassie's face. I shouted something, but I don't think either of them heard.

Footsteps thumped behind me and Sweeney streaked past, running like a rugby player and already pulling out his handcuffs. He grabbed Rosalind by the shoulder, spun her around and slammed her against the wall. Cassie had caught her barefaced and with her hair pulled back in a bun, and for the first time I saw in starkly allegorical relief how ugly she was, without the layered makeup and the artfully tumbling ringlets: pouched cheeks, thin avid mouth pursed into a hateful smirk, eyes as glassy and empty as a doll's. She was wearing her school uniform, shapeless navy-blue skirt and a navy-blue blazer with a crest on the front, and for some reason this disguise seemed to me the most horrible one of all.

Cassie stumbled backwards, caught herself against a tree trunk and regained her balance. When she turned towards me all I could see at first was her eyes, huge and black and blind. Then I saw the blood, a crazy web of it streaking one side of her face. She swayed a little, under the blurred shadows of the leaves, and a bright drop fell into the grass at her feet.

I was only a few yards from her, but something stopped me from moving closer. Dazed and unstrung, her face branded with those fierce markings, she looked like some pagan priestess emerging from a rite too bright and merciless to be imagined: still half somewhere and someone else, and not to be touched until she gave the sign. The nape of my neck prickled.

"Cassie," I said, and held out my arms to her. My chest felt as if it was bursting open. "Oh, Cassie."

Her hands came up, reaching, and for an instant I swear her whole body moved towards me. Then she remembered. Her hands dropped and her head went back, her gaze skidding aimlessly across the wide blue sky.

Sam shoved me out of the way and pounded to a clumsy stop beside her. "Ah, God, Cassie…" He was out of breath. "What did she do to you? Come here."

He pulled out his shirttail and blotted gently at her cheek, his other hand cupping the back of her head to steady her. "Ow. Fuck," Sweeney said, through gritted teeth, as Rosalind stamped on his foot.

"Scratched me," Cassie said. Her voice was terrible, high and eerie. "She touched me, Sam, that thing touched me, Jesus, she spat-Get it off me. Get it off."

"Shhh," Sam said. "Shhh. It's over now. You did great. Shhh." He put his arms around her and pulled her close, and her head went down on his shoulder. For a second Sam's eyes met mine squarely; then he looked away, down at his hand stroking her tumbled curls.

"What the hell is going on?" demanded O'Kelly, behind me, in disgust.

Cassie's face, once it was cleaned up, was not as bad as it had initially looked. Rosalind's nails had left three wide dark lines scored across her cheekbone, but in spite of all the blood they weren't deep. The tech, who knew first aid, said that there was no need for stitches and that it was lucky Rosalind had missed the eye. He offered to put Band-Aids on the cuts, but Cassie said no, not until we got back to work and she had them disinfected. She was shuddering all over, off and on; the tech said she was probably in shock. O'Kelly, who still looked baffled and slightly exasperated by this whole day, offered her an iced caramel. "Sugar," he explained.

She was obviously in no state to drive, so she left her Vespa where it was parked and rode back to work in the front of the van. Sam drove. Rosalind went in the back, with the rest of us. She had settled down once Sweeney got the cuffs on her; she sat rigid and outraged, not saying a word. Every breath I took smelled of her cloying perfume and of something else, some overripe taint of rot, rich and polluting and possibly imaginary. I could tell from her eyes that her mind was working furiously, but there was no expression on her face; no fear or defiance or anger, nothing at all.

By the time we got back to work O'Kelly's mood had improved considerably, and when I followed him and Cassie into the observation room he didn't attempt to send me away. "That girl reminds me of a young fella I knew in school," he told us reflectively, as we waited for Sam to finish going through the rights sheet with Rosalind and bring her up to the interview room. "Shaft you six ways till Sunday without blinking an eye, then turn around and have everyone convinced it was all your own fault. There's mad people out there."

Cassie leaned back against the wall, spat on a bloodstained tissue and scrubbed again at her cheek. "She's not mad," she said. Her hands were still shaking.

"Figure of speech, Maddox," O'Kelly said. "You should go get the war wound seen to."

"I'm fine."

"Fair play to you, all the same. You were right about that one." He clapped her awkwardly on the shoulder. "All that about making the sister sick for her own good; would you say she actually believes that?"