‘At the moment, I’m busy trying to imagine a famous clue,’ DS Malik offered with a wry smile.

‘Well, this one’s a delight. Because if you look at the surface meaning of “Two girls, one on each knee”, it maybe conjures up an image of a bawdy nightclub with lewd strippers on a businessman’s lap. Or a loving parent with young daughters in a homely domestic setting. But the answer’s got nothing to do with what it appears to be about.’

‘Okay, so what’s the answer?’ DS Malik asked.

‘In this instance, the first half of the clue is the wordplay. You’re looking for the names of two girls, each short enough that when you put them together you get a seven-letter word for something that you have one of on each knee.’

‘Just so you know,’ Becks said, ‘I’m still not following any of this.’

‘Don’t worry, I’m only saying there has to be a logical explanation for why the intruder broke into Stefan’s house. We’ve got the surface meaning. He chiselled away a picture frame and then ran off with it. But let’s not worry too much that it appears to make no sense, we simply need to break it down. Work out what the rules were that he was following, and I’m sure we can work out what the answer is. For example, maybe it connects with Stefan being a conman.’

‘What’s that?’ DS Malik asked.

Judith explained how Becks had started a thread on an online forum and garnered a response that said that while people liked Elliot, his father had been in cahoots with Stefan, who’d apparently been something of a crook.

‘How was Stefan a crook?’ DS Malik asked.

‘We don’t know,’ Becks said. ‘The person online didn’t say. They just said he ran scams with Elliot’s father.’

‘Okay, that’s something I can get my team to check out.’

A silence descended on the women as they looked at the frameless painting. Why on earth had the intruder stolen the frame but not the picture?

DS Malik’s face suddenly lit up.

‘Patella!’ she said.

‘I’m sorry?’ Becks said.

‘I’ve got it. The names of two girls. Pat and Ella. And when you put them together, you get “patella”. You have one of those on each knee. “Two girls, one each knee.” Patella.’

‘Bravo!’ Judith exclaimed, delighted.

‘I still don’t get it,’ Becks said.

‘Don’t worry, I’ll explain later.’

‘That’s very clever,’ DS Malik said as she got a pair of evidence gloves from her pocket and put them on. ‘Anyway, I think it’s time we looked at this painting more closely. Don’t you?’

DS Malik lifted the frameless painting from the wall and angled it so they could see it better in the light.

‘It’s not signed by the artist,’ Becks observed.