‘Where are you, Sean?’ Silva asked. From the delay and crackle on the line she suspected he was using a satellite phone and calling from somewhere remote. She felt her defences rise as if she needed to protect herself from something. Almost unconsciously she hardened the tone of her voice. ‘And what do you want?’

‘I’m in Plymouth, Becca. And I want to see you.’

The first week of Holm’s pretend investigation panned out pretty much as he expected. Colleagues poked their heads round the door of the little office to see what he was up to, Huxtable nodded with approval when she saw the fake brief Holm had written on animal rights groups, and Javed continued to slurp his coffee, crunch his biscuits and clip his nails in a way that annoyed Holm immensely.

The two of them monitored the Twitter account of the mysterious user known as TCXGP1505, while Holm set up the dummy animal rights operation. He phoned his contacts and let them know what he was up to and made requests for information from various agencies. A policy book was created and a budget drawn up. Javed scoured the internet for extreme material and organised everything in a database.

The initial excitement Holm had felt when Javed had shown him the tweet slowly ebbed away though. TCXGP1505 remained silent. The account followed nobody and had no followers. The sole tweet was the one in Arabic that referenced the innocent one.

‘I don’t get it,’ Holm said. ‘I’m beginning to wonder if our source, if that’s what it was, has been compromised.’

Javed looked up from his screen, horrified. ‘You mean taken out?’

‘It figures. What else can explain the lack of a follow-up?’

Javed shrugged. ‘If it’s somebody close to Taher then the risk must be enormous. Perhaps they got cold feet.’

‘Perhaps.’ But Holm didn’t think so. They’d sent one message. The tease. The wake-up call. The bait. If the source was still alive there had to be another one along at some point.

And so it proved.

On Friday afternoon, as Holm was deciding whether to take up Palmer’s offer of a couple of jars after work, Javed lurched back in his chair.

‘We’re on!’ The chair rocked violently as Javed changed direction and hunched forward, his face inches from the screen. ‘What the…?’

Holm stood and moved over. The tweet was a sequence of numbers, seventeen in all.

‘18, 18, 14, 0, 21, 11…’ Javed began to read them. He turned from the screen. ‘Another code.’

‘Makes sense, after all, it’s what we spooks do, isn’t it? Codes and dead-letter drops and listening to phone conversations. Least that’s what I believed before I signed on. More fool me.’