And, more importantly, by listening.

She had reached a busier part of the river, by Mill Lane. Up ahead, people were walking, running, biking. Mariana contemplated them. The killer could be any one of these people. He could be standing here right now.

He could be watching her.

How would she recognise him? Well, the simple answer was she couldn’t. And despite all of Julian’s claims of expertise, he couldn’t either. Mariana knew that, if asked about psychopathy, Julian would point to frontal or temporal lobe damage in the brain; or quote a series of meaningless labels – antisocial personality disorder, malignant narcissism – along with a glib set of characteristics like high intelligence, superficial charm, grandiosity, pathological lying, a contempt for morality – all of which explained very little. It didn’t explain how – or why – a person might end up like this: as a merciless monster, using other human beings as if they were broken toys to be smashed to bits.

A long time ago, psychopathy used to be called simply ‘evil’. People who were evil – who took a delight in hurting or killing others – were written about ever since Medea took an axe to her children, and probably long before that. The word ‘psychopath’ was coined by a German psychiatrist in 1888 – the same year Jack the Ripper terrorised London – from the German word psychopastiche, literally meaning ‘suffering soul’. For Mariana this was the clue – the suffering – the sense that these monsters were also in pain. Thinking about them as victims allowed her to be more rational in her approach, and more compassionate. Psychopathy or sadism never appeared from nowhere. It was not a virus, infecting someone out of the blue. It had a long prehistory in childhood.

Mariana believed that childhood was a reactive experience, meaning that in order to experience empathy for another human being, we must first be shown empathy – by our parents or caregivers. The man who killed Tara was once a little boy – a boy who was shown no empathy, no kindness. He had suffered – and suffered horribly.

Yet many children grow up in terribly abusive environments – and they don’t end up as murderers. Why? Well, as Mariana’s old supervisor used to say, ‘It doesn’t take much to save a childhood.’ A little kindness, some understanding or validation: someone to recognise and acknowledge a child’s reality – and save his sanity.

In this case, Mariana suspected there had been no one – no kindly grandmother, no favourite uncle, no well-meaning neighbour or teacher to see his pain, name it, and make it real. The only reality belonged to his abuser, and the small child’s feelings of shame, fear, and anger were too dangerous to process alone – he didn’t know how – so he didn’t process these feelings; he didn’t feel them. He sacrificed his true self, all that unfelt pain and anger, to the Underworld, to the murky world of the unconscious.

He lost touch with who he really was. And the man who lured Tara to that isolated spot was a stranger as much to himself as he was to everyone else. He was, Mariana suspected, a brilliant performer: impeccably polite, genial and charming. But Tara provoked him somehow – and the terrified child inside him lashed out, and reached for a knife.

But what had triggered him?

That was the question. If only Mariana could see into his mind and read his thoughts – wherever he was.

‘Hello there.’

The voice behind her made Mariana jump. She quickly turned around.

‘Sorry,’ he said. ‘I didn’t mean to scare you.’

It was Fred, the young man she’d met on the train. He was pushing a bicycle, with a stash of papers under his arm, eating an apple. He grinned.

‘Remember me?’

‘Yes, I remember.’

‘Said we’d meet again, didn’t I? I predicted it. Told you, I’m a bit psychic.’

Mariana smiled. ‘Cambridge is a small place. It’s a coincidence.’

‘Take it from me. As a physicist. No such thing as coincidence. This paper I’m writing here actually proves it.’

Fred nodded at his stack of papers, which slipped out from under his arm – and pages of mathematical equations cascaded all over the path.

‘Bugger.’

He threw down his bike to the ground, and ran around trying to retrieve the pages. Mariana knelt down to help.

‘Thanks,’ he said as they collected the last of the pages.

He was inches away from her face, staring into her eyes. They looked at each other for a second. He had nice eyes, she thought, before banishing the thought. She stood up.

‘I’m glad you’re still here,’ he said. ‘Will you be staying long?’

Mariana shrugged. ‘I don’t know. I’m here for my niece – she – she’s had some bad news.’

‘You mean the murder? Your niece is at St Christopher’s, right?’

Mariana blinked, confused. ‘I – don’t remember telling you that.’

‘Oh – well, you did.’ Fred went on quickly. ‘Everyone’s talking about it – about what happened. I’ve been thinking about it a lot. I have a few theories.’

‘What kind of theories?’

‘About Conrad.’ Fred glanced at his watch. ‘I’ve got to run right now, but I don’t suppose you fancy having a drink? Say – tonight? We could talk.’ He looked at her hopefully. ‘I mean, only if you want to – obviously, no pressure – no big deal …’

He was tying himself into knots; Mariana was about to refuse and put him out of his misery. But something stopped her. What did he know about Conrad? Perhaps Mariana could pick his brains – he might know something useful. It was a worth a shot.

‘Okay,’ she said.

Fred looked surprised and excited. ‘Really? Fantastic. How about nine o’clock? The Eagle? Let me give you my number.’

‘I don’t need your number. I’ll be there.’

‘Okay,’ he said with a grin. ‘It’s a date.’

‘It’s not a date.’

‘No, of course not. I don’t know why I said that. Okay … See you later.’

He got on his bike.

Mariana watched Fred cycle away along the river path. Then she turned and started walking back to college.

Time to begin. Time to roll up her sleeves and get to work.

4

Mariana hurried across Main Court, towards a group of middle-aged women, all sipping tea from steaming mugs, sharing biscuits, and gossiping. These were the bedders – on their tea break.

‘Bedder’ was a term peculiar to the university, and something of an institution – for hundreds of years, armies of local women had been employed at the colleges to make beds, empty rubbish bins, and clean rooms – although, it must be said, the bedders’ daily contact with the students meant the role often crossed over from domestic service to pastoral care. Mariana’s bedder was sometimes the only person she spoke to every day, until she met Sebastian.

The bedders were a formidable bunch. Mariana felt a little intimidated as she approached them. She wondered – not for the first time – what they really thought of the students; these working-class women who had none of the advantages of these privileged, often spoiled young people.

Perhaps they hate us all, Mariana suddenly thought. She wouldn’t blame them if they did.

‘Good morning, ladies,’ she said.

Their conversations faded to silence. The women gave Mariana a curious and slightly suspicious look. She smiled.

‘I wonder if you might be able to help me. I’m looking for Tara Hampton’s bedder.’

Several heads turned towards one woman who was standing at the back, lighting a cigarette.

The woman was in her late sixties, possibly older. She was wearing a blue smock, and carrying a bucket of various cleaning products and a feather duster. She was not plump, but stolid and moon-faced. Her hair was dyed red, white at the roots, and her eyebrows were painted on daily; today she had drawn them high on her forehead, making her look rather startled. She seemed a little irritated to have been singled out. She gave Mariana a strained smile.

‘That’ll be me, dear. I’m Elsie. How can I help?’

‘My name’s Mariana. I was a student here. And I …’ she went on, improvising, ‘I’m a psychotherapist. The dean has asked me to talk to various members of college about the impact of Tara’s death. I was wondering if we might … have a little chat.’

She finished lamely, and didn’t hold out much hope that Elsie would take the bait. She was right.

Elsie pursed her lips. ‘I don’t need a therapist, dear. Nothing wrong with my head, thank you very much.’

‘I didn’t mean that – it’s for my own benefit, really. It’s – I’m conducting some research.’

‘Well, I really don’t have the time—’

‘It won’t take long. Perhaps I can buy you a cup of tea? A slice of cake?’

At the mention of cake, a glint appeared in Elsie’s eye. Her manner softened. She shrugged, and took a drag of her cigarette.

‘Very well. We’ll have to be quick. I’ve another staircase to do before lunch.’

Elsie stubbed out her cigarette on the cobbles, then pulled off her apron and thrust it at another bedder, who took it wordlessly.

Then she walked over to Mariana.