There’s no mystery,” Detective Sergeant Siobhan Clarke said. “Herdman lost his marbles, that’s all.”

She was sitting by a hospital bed in Edinburgh ’s recently opened Royal Infirmary. The complex was to the south of the city, in an area called Little France. It had been built at considerable expense on open space, but already there were complaints about a lack of useable space inside and car-parking space outside. Siobhan had found a spot eventually, only to discover that she would be charged for the privilege.

This much she had told Detective Inspector John Rebus on her arrival at his bedside. Rebus’s hands were bandaged to the wrists. When she’d poured him some tepid water, he’d cupped the plastic glass to his mouth, drinking carefully as she watched.

“See?” he’d chided her afterwards. “Didn’t spill a drop.”

But then he’d spoiled the act by letting the cup slip as he tried to maneuver it back on to the bedside table. The rim of its base hit the floor, Siobhan snatching it on the first bounce.

“Good catch,” Rebus had conceded.

“No harm done. It was empty anyway.”

Since then, she’d been making what both of them knew was small talk, skirting questions she was desperate to ask and instead filling him in on the slaughter in South Queensferry.

Three dead, one wounded. A quiet coastal town just north of the city. A private school, taking boys and girls from ages five to eighteen. Enrollment of six hundred, now minus two.