Thorne and Kambar began to eat.

‘Is it possible?’ Thorne asked. ‘This change of personality business.’

‘Oh, personality change is certainly possible. I’ve dealt with a number of cases. But to the degree where you might murder someone? ’

‘Where you might murder seven someones.’

‘This is almost a Jekyll and Hyde thing we’re talking about.’

‘So?’

‘I was… dubious.’

‘You’re not saying it isn’t feasible, then?’

‘Almost nothing is hard and fast where the brain is concerned,’ Kambar said. ‘It’s nigh-on impossible to rule out anything completely, but there was no way I would have been willing to say that in a court of law.’

Thorne began picking up chips with his fingers. ‘I think I get it,’ he said.

‘Good. The lasagne’s better than normal today. It’s usually solid.’

Thorne knew plenty of doctors and scientists who would have trotted happily up to the witness stand in search of notoriety or a hefty fee. Who would have said that, although such a thing were unlikely, they could not say for certain that it had not happened. People of that sort – many of whom were virtually professional expert witnesses – were gifts to defence barristers seeking to get the likes of Raymond Garvey off the hook. Such testimony was almost designed to plant the seed of reasonable doubt within the mind of even the most sceptical juror.

The relatives of those murdered by Garvey should have been very grateful to Pavesh Kambar.

‘These cases you’ve dealt with,’ Thorne said, ‘how do these changes happen?’

Kambar raised his hand to demonstrate and it looked as though he might stab himself in the forehead, until he remembered and put down his fork. ‘The frontal lobe is what controls our cognition,’ he said. ‘It’s where the brain’s natural inhibitors are, where all the levels are set. It’s what makes us who we are.’

‘And a tumour can change that?’

‘Any foreign body, or any injury that affects that area. If the brain gets damaged, the personality can be affected. Altered.’

‘I read something in a paper once,’ Thorne said. ‘This woman suffered a massive head injury in a car accident and when she woke up she was speaking in a completely different language.’

Kambar nodded. ‘I’ve seen similar cases reported,’ he said. ‘But I’m not convinced. I think those kinds of things make good stories.’

‘So, what sorts of changes have you seen?’

‘Shy people who can suddenly become extremely gregarious. It’s usually a question of inhibition, of barriers coming down. Alcohol works in the same way in that it disinhibits the frontal lobe. Imagine someone who is very drunk, but without the falling over and the slurred speech. There are no… niceties, you know? Social graces go out of the window, the mark is overstepped.’

‘I’ve seen that,’ Thorne said.

Kambar shoved the last forkful of pasta into his mouth and waited.

Ignoring what was left of his lunch, Thorne found himself telling this man he had known for only an hour about the Alzheimer’s that had blighted his father’s final years and a few of his own. About the old man’s bizarre obsessions and the lifestyle that had grown increasingly erratic and disturbing. Kambar told him that the disease acted on the brain in precisely the way he had been describing.

‘People think it’s all about forgetting people’s names or where you’ve left your keys,’ Kambar said. ‘But the worst thing is that you forget how to behave.’

Thorne laid down his cutlery. Straightened it. ‘What about the whole genetic thing?’

Kambar nodded, understanding what he was being asked. ‘Look, it’s far from being definitive, but only something like fifteen per cent of patients with Alzheimer’s had parents who suffered from it; and even then the strongest genetic link is with the rarest forms, like early onset. We’re not talking about that, right?’

Thorne shook his head.

‘The fact that your father had it might increase your own susceptibility a little, but no more than that.’ Kambar smiled. ‘Dementia is very common, though, and chances are you’ll get it anyway, so I’d stop worrying. ’

‘Sometimes it was good,’ Thorne said. ‘With my dad, you know? There was this one afternoon we were all playing bingo on the pier and he just lost it. Started swearing and shouting, proper filth, and everyone was upset, but I was pissing myself. And he knew it was funny. I could see it in his face.’

‘I’m glad it wasn’t all gloom and doom,’ Kambar said. ‘How was it at the end?’

Thorne suddenly found his appetite again. He had discovered only recently how the fire in which Jim Thorne perished had started; the part he had played in the death of his own father. He had not even felt able to share the truth with Louise. He heard Kambar from the other side of the table telling him that it wasn’t a problem, that he had not meant to pry.

Thorne started slightly when Kambar’s beeper went off. He got up and shook the doctor’s hand when it was offered. ‘You’ve been a great help. Thank you.’

‘I wish I could tell you I was off to perform some vital brain surgery,’ Kambar said. ‘But the truth is I’ve got a squash game.’ He reached inside his jacket and rubbed his stomach. ‘Should have eaten lunch a bit earlier.’

‘That was my fault.’

‘It’s not a problem.’

‘Someone’s killing the children of his victims,’ Thorne said suddenly.

‘Sorry?’ Kambar pulled his cryptic crossword face again.

Thorne could see a small blob of sauce at the edge of the doctor’s moustache, a thin streak of it just below his collar. ‘The children of the women that Raymond Garvey murdered.’ Thorne suddenly felt a little dizzy and guessed he’d stood up too quickly. He took a couple of seconds, hoping that Kambar would think the pause was for his benefit. ‘Whoever had those fragments of Garvey’s brain scan has already killed four people.’

Kambar looked as though he wished he had never asked. He puffed out his cheeks, said, ‘Fuck.’

The surprise was clearly evident on Thorne’s face.

‘It’s a medical term,’ Kambar said. ‘One you reserve for when you hear something that makes you feel like a hopeless quack with a pocketful of leeches.’

‘I use it pretty much the same way,’ Thorne said. ‘Just more often.’

‘There are so many things that can mess up the brain, but most of them we can do nothing about.’ Kambar shook his head, the resignation etched in lines around his mouth. ‘Sometimes the damage is… invisible.’

‘Enjoy your game,’ Thorne said.

When the doctor had gone, Thorne walked over to the counter again. He bought a coffee and a thick slice of cheesecake, took them back to the table. From the window, there was a spectacular view across the flat, green fenland: Grantchester huddled a little to the north; the spires of Cambridge just visible a few miles away to the east; and the pulsing grey vein of the M11 halfway to the horizon.

Thorne looked out, savoured his dessert and tried to remember exactly what his father had shouted that day on the pier. Based on what Kambar had told him, his father could probably have committed murder with a fair chance of getting away with it. It’s a shame his dad had never known that. He was a crotchety and unforgiving old sod sometimes, especially in the last few years. He’d probably have drawn up a decent-sized hit list.

‘Garvey’s son thinks his father was wrongly imprisoned, and that the tumour might have been found earlier if he hadn’t been in prison. So he blames the world and his wife for his father’s death.’

‘I’m still not convinced this nutcase is Garvey’s son,’ Thorne said.

‘Sounds like Garvey was, though.’

‘OK, for the sake of argument…’

‘So, the child of the killer starts killing the children of the victims. It makes a kind of sense when you think about it.’

‘Sense?’ Thorne said.

‘You know what I mean.’