McNab had required a second pint before he’d got down to the job of calling the professor. (He would have preferred whisky but was still rationing himself on that front.)

Pirie, acting surprised to hear McNab’s voice, had been cautious but positive in his response.

‘Are you contacting Mr McLaughlin or shall I?’ he’d said.

‘DS Clark has already been in touch with him,’ McNab had responded. What he didn’t repeat were DS Clark’s words: ‘McLaughlin was very keen, and stressed he’d like Professor Pirie to be there.’

‘So I’m officially on the case?’ Magnus had said, sounding self-satisfied to McNab’s ear.

‘You’re on for McLaughlin’s interview,’ McNab had conceded, knowing full well that meant the boss wanted the professor fully involved. ‘Can you be here tomorrow morning at nine?’

McNab had chosen the time, hoping that Professor Pirie might well have a class then. There had been a moment’s pause, in which McNab celebrated the fact that the professor wouldn’t make it after all, before his hopes had been crushed.

‘That’s fine, Detective Sergeant. Shall I come in earlier to discuss tactics?’

Discuss tactics? Fuck’s sake, McNab had almost said, but had managed not to.

The time they’d agreed was fast approaching. McNab had had to miss his workout this morning just to meet it. One bonus at least.

As he contemplated this, DS Clark arrived with the professor and, thankfully, coffee.

‘You’ll be behind the observation glass, Professor,’ McNab said. ‘No doubt your star pupil will be giving his best performance.’

‘Are you planning a standard cognitive interview focusing on the day of the girl’s disappearance?’ Pirie asked.

‘Yes,’ Janice came in quickly. ‘We’ll start with free recall around the day Mary McIntyre disappeared. We have some images of the street layout to help with context reinstatement.’ She halted there, noting McNab’s expression.

‘I expect McLaughlin will know all about this method, Professor,’ McNab said, ‘seeing that his degree is in Forensic Psychology?’

‘He will,’ Pirie nodded, with no sense of taking offence at McNab’s obvious dig, ‘and in depth.’

‘So, Professor,’ McNab said. ‘What do you suggest?’

‘Mr McLaughlin loves the sound of his own voice so getting him to talk won’t be a problem. However, despite his knowledge of psychology, I suspect getting him to recall backwards rather than in chronological order, since lies are usually rehearsed that way, may prove useful. Another thing. He’s narcissistic, so perhaps get him to try and recall the day from someone else’s point of view. Say Karen Marshall’s, Mary’s best friend. The girls were apparently joined at the hip, metaphorically speaking.’

‘How did you know that?’ McNab said, recalling Robbie, Mary’s brother, using the same expression.

‘I’ve been reading old newspaper reports,’ Magnus told him. ‘Why?’

‘Nothing,’ McNab said as his mobile buzzed. Answering, he listened, then said, ‘Someone will be up for him.’ Ringing off, he looked at Janice. ‘Me or you?’

‘I’ll go,’ Janice said. ‘Let’s see how he deals with a female officer. Fifteen years ago there weren’t many of us around.’