It was the data analyst from GCHQ. A slim woman with thin lips and no jewellery that Luke could see. He put her age at about thirty-five.

‘Well …’ She hesitated. Luke wondered if she already regretted speaking up. ‘They do have a point there, don’t they?’ Silence in the room as all eyes turned towards her. ‘I mean,’ she pressed on, ‘Germany did let in one and a half million migrants – sorry, refugees – from Syria in 2015. That’s surely changed the character of a whole nation and—’

‘Okay, let me stop you there.’ Louise Band took a few paces towards her and she was wagging her finger. ‘This is a question for another time,’ she said briskly. ‘We are here this morning to address an urgent terrorist threat. A threat from the sort of mindset inhabited by people like Anders Breivik, the man who shot dead dozens of young people on a Norwegian island in 2011. So, if there are no further questions let’s continue.’

That was an odd intervention, Luke thought. He exchanged a discreet glance with Jenny, then took a long look at the data analyst. He noticed she was staring at her notebook and blushing.

‘Right, so, on top of the great replacement theory,’ continued the MI5 briefer, ‘you’ve got the white genocide theory. This was something the neo-Nazis in the US got exercised about, back in the seventies. They reckoned there was an actual plot to wipe out white people through racial integration. I know, I know,’ she put up her hands theatrically, ‘sounds crazy, right? But these ideas are coming back and they’re being propagated daily through online manifestos. Who in here has heard of 8Chan?’

Everyone nodded, including the Norwegian intelligence officer sitting next to Luke.

‘All of you? Good. So before it got closed down, 8Chan was the web forum where a lot of people shared their extreme ideas and even posted manifestos. And here there are definite commonalities with ISIS in the way they put out their propaganda. They create content – extremist material, if you will – and they put it out on social-media platforms.’

Luke noticed Jenny was busy scribbling everything down while he was trying his best to commit all this to memory. He was certain she would have come top of every class at school, a sort of latterday Hermione Granger. And there, sitting quietly on the opposite side of the horseshoe table, was John Friend, the Service lawyer, also scribbling diligently away and constantly looking something up in a pamphlet he kept retrieving from his inside jacket pocket.

‘When people comment on this material on social media,’ the briefer was saying, ‘that’s when they try to lure them on to the encrypted apps. And that is where the conversations take on a more violent turn – you know, people talking about what they’d like to do to others.’