She was too late. Doireann Greaney ran straight up to the young woman without slackening her pace and pushed her backwards off the wall. The young woman must have had her dog’s lead tied to her wrist because the terrier was plucked up off the pavement like a conjuring trick and disappeared after her.

Kyna reached the wall seconds later – in time to hear the young woman splash into the river only three metres below and scream out for help.

She turned to face Doireann Greaney, and for two or three seconds they stared at each other. Underneath her grey hood, Doireann Greaney was thin-faced, sallow, with a pointed nose and unplucked eyebrows and huge dark eyes, but she was strangely attractive, in the same way that witches or banshees could be attractive. She was breathing hard but she said nothing.

‘Stay right where you are, Doireann,’ Kyna snapped at her. ‘I’m a police officer and you’re under arrest. I mean it. Don’t fecking move a muscle.’

The young woman screamed again. Kyna could see that Detective Cushley had parked on the opposite side of French’s Quay and was climbing out of the car to come and help her. Without any further hesitation, she unbuttoned her coat and tugged it off, and then her jacket, and kicked off her boots. She swung her legs over the wall and carefully climbed down the craggy limestone embankment until she reached the river’s edge.

The young woman was about twenty metres away by now, the tails of her red raincoat curving out on either side of her like wings. She was thrashing her arms to stay afloat, although every few seconds her head disappeared under the surface, and when she reappeared she spat out water and coughed and tried to scream.

Beside her, her terrier was furiously paddling, but one of its legs was tangled up in its lead, and so it was having difficulty in keeping itself upright.

‘Sarge!’ shouted Detective Cushley. Kyna looked up and saw him leaning over the wall. He had untied one of the red-and-white lifebelts from the railings along the other side of the bridge. He dropped it down and it bounced off the stones at the brink of the river and slowly floated towards the young woman. The tide was going out, and she had already been drawn downriver until she was underneath the bridge.

‘Hold on, sarge! I’ve called for backup!’ Detective Cushley told her.

The young woman went under again. Kyna took a deep breath and dived into the river. She had always been a strong swimmer, but the water was stunningly cold – so cold that she felt as if she had been electrocuted and gone into shock. She struggled to the surface, gasping to get her breath back, but ripples from the river smacked her across the face, again and again, like a petulant lover, as if it were punishing her for diving into it.