‘Coffee.’

‘No, thank you.’ Kelly hadn’t eaten all day but she didn’t think she could stomach anything. Diggers had hung around for half an hour after dismissing her, before disappearing to do whatever a nearly retired DCI did with an accumulation of rest days in lieu. He hadn’t spoken to Kelly again; only paused by Nick’s desk on his way out, for a muttered conversation Kelly had been certain was about her.

‘It wasn’t a suggestion,’ Nick said. ‘Get your coat, we’re going across the road.’

The Starbucks on Balfour Road was more of a takeaway than a café, but it boasted two high stools in the window, which Kelly commandeered, while Nick got the drinks. Kelly ordered a hot chocolate, suddenly craving its sweet comfort. It arrived topped with whipped cream and sprinkled with chocolate, looking embarrassingly gauche next to Nick’s flat white.

‘Thank you,’ Kelly said, when it became clear Nick wasn’t going to do the talking.

‘You can get the next ones,’ he said.

‘For bailing me out, I mean.’

‘I know what you meant.’ He fixed her with an unsmiling gaze. ‘For future reference, if you fuck up, or you do something stupid, or for some other reason you’re likely to need bailing out, for God’s sake tell me. Don’t wait until we’re sitting in the DCI’s office.’

‘I really am sorry.’

‘I’m sure.’

‘And very grateful. I didn’t expect you to do that.’

Nick took a sip of his coffee. He grinned. ‘To be honest, I didn’t expect me to, either. But I couldn’t sit by and see one of the best detectives I’ve worked with’ – Kelly looked down at her hot chocolate to hide how pleased she was – ‘get the boot for doing something so monumentally stupid as to use her position for some sort of personal campaign. What exactly were you doing?’

The pleasurable flush Kelly had felt at Nick’s compliment disappeared.

‘I think an explanation is the least you owe me.’

Kelly spooned some of the warm cream into her mouth, feeling it dissolve on her tongue. She tested the words out in her head before she spoke. ‘My sister was raped in her first year at Durham University.’

‘That much I gathered. And the offender was never caught?’

‘Never. There had been several suspicious incidents prior to the rape; Lexi found cards in her pigeonhole asking her to wear certain clothes – outfits she had in her wardrobe – and once someone left a dead goldfinch outside her door.’

‘Did she report it?’

Kelly nodded. ‘The police weren’t interested. Even when she told them she was being followed they just said they’d make a note of it. She had a late lecture on a Thursday evening and no one else walked back the same way as her, so she was on her own. The night it happened she was on the phone to me. She called because she was feeling nervous – she said she could hear footsteps behind her again.’

‘What did you do?’

Kelly felt her eyes burn with the threat of tears. She swallowed hard. ‘I told her she was imagining it.’ She could hear Lexi’s voice, even now; breathless as she walked to halls.

‘There’s someone behind me, Kelly, I swear. Just like last week.’

‘Lex, there are seventeen thousand students at Durham – there’s always someone behind you.’

‘This is different. They’re trying not to be seen.’ Lexi spoke in an urgent whisper, Kelly straining to hear every word. ‘When I turned round just now there was no one walking, but they’re there, I know it.’

‘You’re getting yourself in a state. Give me a call when you get home, yeah?’

Kelly had been getting ready to go out, she remembered. She’d cranked the music up as she did her hair; threw another rejected dress on the pile at the end of the bed. It never crossed her mind that Lexi hadn’t called, until her mobile had rung with a number she hadn’t recognised.

‘Kelly Swift? This is DC Barrow-Grint from Durham police. I’ve got your sister with me.’

‘It wasn’t your fault,’ Nick said gently. Kelly shook her head.

‘He wouldn’t have attacked her if I’d stayed on the phone.’

‘You don’t know that.’

‘If he had, I’d have heard – I’d have been able to call the police straight away. It was two hours before Lexi was found – she’d been beaten up so badly she could hardly see – and by that time the offender was long gone.’

Nick didn’t contradict her. He turned his coffee cup around in its saucer until the handle was facing him, cupping both hands around it. ‘Does Lexi blame you for what happened?’

‘I don’t know. She must do.’

‘You haven’t asked her?’

‘She won’t talk about it. Hates it when I do. I thought she’d be affected for months – for ever, even – but it was as though she just drew a line under the whole thing. When she met her husband she sat him down and said “There’s something you need to know”, and she told him the whole story then made him promise never to mention it again.’

‘She’s a strong woman.’

‘You think so? I don’t think it’s healthy. Pretending something didn’t happen isn’t the way to deal with a traumatic event.’

‘You mean, it’s not the way you deal with traumatic events,’ Nick said.

Kelly looked at him sharply. ‘This isn’t about me.’

Nick drained his coffee and set the cup carefully on the saucer before looking Kelly in the eye. ‘Exactly.’

Kelly’s mobile rang as they returned to work. She hung back at the top of the stairs, avoiding the noise of the busy MIT office. It was Craig, from the CCTV hub.

‘Kelly, have you seen BTP’s internal briefing today?’

She hadn’t. It was hard enough to keep up with the volume of emails relating to this job, without reading her own force’s daily missives.

‘The CCTV room here has been compromised. Given what you told me the other day about your Met job, I thought I should give you a ring.’

‘A break-in?’

‘Worse. A hacking.’

‘I thought that was impossible?’

‘Nothing’s impossible, Kelly, you should know that. The system’s been sluggish for a few weeks; we called an engineer and when he came to take a look he identified some malware. We’ve got a firewall in place which makes it nigh-on impossible to be hacked over the web, but doesn’t stop someone physically introducing viruses to the system.’

‘An inside job, then?’

‘All the staff were interviewed in turn this morning by the superintendent, and one of the cleaners broke down. Said she’d been bribed to carry a USB stick in and put it in the main computer. Of course she claims to have had no idea what she was doing.’

‘Bribed by whom?’

‘She doesn’t know his name, and conveniently doesn’t remember what he looks like. She says she was approached on the way to work one day and offered more than a month’s salary for a few minutes’ work.’

‘What’s the extent of the hacking?’

‘The malware introduces a programme which talks to the hacker’s computer and replicates the entire system. They can’t control the camera direction, but the bottom line is whatever our control room sees; the hacker can see.’

‘Oh my God.’

‘Does it fit with what you’re dealing with?’

‘It’s certainly possible.’ Despite her good working relationship with Craig, Kelly was mindful of what Diggers might say, if she were to release any more information than necessary. The last thing she needed was another telling off, although there was no doubt in her mind the two jobs were related.

‘Our offender’s been using London Underground’s own cameras to stalk women,’ Kelly announced, walking into the office and interrupting a conversation Nick was having with Lucinda. She filled them in on the call from Craig. ‘BTP’s Cyber Crime unit are there now, but although they’ve identified the malware, it’s less straightforward to eradicate it.’

‘Couldn’t they switch off the whole system?’ Lucinda asked.

‘They could, but then the entire city would potentially be at risk, instead of—’

‘Instead of a small number of women definitely at risk,’ Nick finished. ‘We’re between a rock and a hard place.’ He stood up, his whole body energised, and Kelly realised how much he thrived on the adrenaline of a fast-moving investigation. ‘Right, we need a statement from your CCTV contact, and I want that cleaner nicked for unauthorised access to computer systems with intent to commit crime.’ He looked around for the HOLMES loggist, who was already entering the actions into the laptop in front of him. ‘And get Andrew Robinson here. I want to know where that CCTV feed is being copied to, and I want to know it now.’