Amy went over to the window and opened it. “Sorry, darling, I can’t bear it these days. No, don’t put it out, I’ll just stay over here.”

If Gideon was right about the glitch trails, it meant there was someone on the ward that night. So why didn’t anyone see anything? Why didn’t the nurse see it, if the someone was right there? Harper thought about the trampled nettles in the bushes opposite Lauren’s window. That time, Patrick hadn’t seen it either. There was a darkness to this, something unknown, the tang of evil.

“What about the intruder?” said Gideon. “I mean, what happened to the patient?”

“I’m not sure I should discuss it with you. I’m sorry.”

Gideon shrugged, turned to Amy. “She’s no fun, is she?”

From the window, Amy said, “How about the other thing, Jo-Jo?”

“There is another thing,” said Harper, “an audio recording. It’s fuzzy, I can’t make out the words on it. Do you think you could sharpen it up?”

“Of course,” said Gideon, “my speciality. If you’ll go through to the lab.”

They climbed the stairs, long since stripped of their carpet and showing several generations of paintwork where many feet had worn through the layers. Gideon led them into one of the four bedrooms, which had been transformed into a music technology suite. It was lined with sound-insulating foam and filled with all manner of gadgetry: keyboards, a huge mixing desk, electronic drum pads, black boxes and racks of hi-fi type equipment, all of it a mystery to Harper. Right in the centre were two computer screens. Gideon took a seat in the swivel chair and pressed four or five buttons to boot the system up.

“Won’t be a moment,” he said as the studio came alive around them, with blinking coloured lights and the hum of several tiny electronic fans. Above the desk there was a square hole in the wall fitted with a thick glass panel through which the neighbouring bedroom could be seen—or what would have been a bedroom, had it contained a bed. Instead there was a drum kit in there, a set of record decks and a selection of guitars hanging on the wall.

“This is also, of course, highly confidential,” she said to him. “I really don’t want you talking to anyone about this.”

“You can trust me,” said Gideon, and in that moment she was surprised to find that she believed him.

She sent the mp3 file of the 999 call to Gideon’s email account and he played it through the huge speakers mounted in the upper corners of the room. Lauren’s terrified voice exploded at them, cutting through Harper painfully in the split second before Gideon reached for the fader to bring the volume down. Even at a low level, the recording was chilling.