Eventually they told her that there was CCTV footage of her with Johnny the previous night. They showed it to her and Lizzie, tired and emotional, almost cried when she saw the fuzzy images of herself and Johnny emerging from the flats and taking the alleyway down to the river. The film switched to another camera and then another. The quality was grainy but she recognised their two figures walking along the Embankment, chatting, stopping to look out across the river.

“The second camera on Paul’s Walk wasn’t working,” one of the police officers said. Cook was a gaunt detective inspector who looked as though he spent too much time indoors staring at computer screens or possibly cadavers. He treated Lizzie with respect and utter courtesy which somehow scared her all the more. “However, there’s another one here—” he stopped the tape and pointed to the screen “—which picked up some images. They aren’t as clear as we would like but they’re interesting.” He pressed play again. “Could you tell us what you were doing here?”

Lizzie watched her own image as she and Johnny stopped in front of the blue plaque that marked the site of Baynard’s Castle.

“I’ve already told you about this,” she said, as patiently as she was able. “Johnny had told me that my flats were built on the site of a medieval palace. He wanted to show me where it had been.”

“It looks as though you’re reaching out for something,” DI Cook said.

“I was touching the plaque that commemorates the site,” Lizzie said.

She leaned in closer, her heart suddenly beating hard in her throat. She had no idea what the camera was going to show. Both she and Johnny were in the frame, their figures close together, blurry but distinct enough to see.

The jerky black-and-white images seemed to freeze for a moment and then there was a flash of bright white light with a darker tinge around the edges, like a firework exploding. Lizzie caught her breath.

DI Cook froze the image. “What happened there?” he said, with deceptive quietness.

Lizzie shook her head. “I don’t know. I don’t remember anything—” She stopped as she caught sight of the fleeting expression of scepticism on DI Cook’s face.

“You hadn’t fainted at this point,” he said, pointing to her figure on the screen, “yet you don’t remember?”

“No,” Lizzie said.

“If my client says she doesn’t remember,” the lawyer put in, “then she doesn’t.”

For a moment Lizzie met DI Cook’s eyes and they were both united in a shared exasperation. He restarted the film. The bright white light expanded, blanking out everything else in the image for a few seconds, and then it died away. Neither Lizzie nor Johnny were visible in the picture anymore. The frame was empty.

Without commenting, DI Cook changed screens to another camera. “This is from the corner of White Lion Hill, where you were found. It’s about twenty yards away and this is just over three minutes later.”