‘The hang-up call on my secret mobile? I called the number back – it was a payphone. The caller didn’t identify themselves and we started to get a bit jittery. But then Mel saw a picture of you on Twitter that some random chav had retweeted and the time corresponded exactly with the mystery call. And hey presto: where were you in the picture? A phone booth in West Hampstead.’

Ryley Warner. He’d taken a picture of me in the phone booth after I’d told him to fuck off, then posted it on Twitter. And then one of his followers had retweeted it to all of their followers, and Mel happened to be one of them. And so it continued: endless, circular, like some mythical beast that ended up consuming itself, an unending sequence of pictures and status updates and tweets and retweets and comments and favourites, all going round in ever-decreasing circles until –

‘You didn’t use a phone booth today, did you?’ Beth continued. ‘There was no reason for you to do that, because you thought you’d found the little spy inside your mobile already. Thought you’d got rid of it and that your mobile was safe to use again.’

‘The app you installed on my phone. System Track or whatever it’s called.’

‘SysAdminTrack,’ she corrected me. ‘Best sixty dollars I ever spent. It’s been on all your phones, by the way: the one you lost at the park, the replacement and the one you’ve got now.’

‘I deleted the app.’

‘You thought you deleted it. It stays in the system memory, hidden. The only way to properly get rid of it – if you’re not the one who downloaded it – is to do a full factory reset or get a new phone.’

‘You sound like an expert on this stuff.’

‘I installed it on Alice’s phone last year when she started going to the park with boys after school. But I’ve always kept up with Ben’s line of work. He never told me anything about what he did, about the industry that made his fortune – our fortune – so I made sure I kept up to speed. Waiting for the day he asked me to join him in the business.’ She frowned. ‘He never did, of course. Never thought I was worthy.’

‘Is that why you killed him?’

She ignored my question.

‘You didn’t call your detective chief inspector, did you?’ Her gaze was cool, unflustered. ‘You’re a terrible liar, Joe. It puts you at a big disadvantage.’

‘I guess I’ll bow to your expertise in that area.’