The Silver Serpent. Lena Jones

boarded the plane?’

      He nods. ‘Dr MacDonald made enquiries with the airport. It was a late-night flight – eleven o’clock – but Sheila never checked in.’

      ‘What about her family?’

      ‘They haven’t heard from her.’

      ‘Why didn’t someone just call the police?’ I ask. ‘It sounds like a straightforward missing person’s case.’

      ‘Ah. The police aren’t convinced there’s “foul play” involved. They say Ms Smith is perfectly within her rights to take off without notifying anyone. They did have a quick check of her flat, and there was no sign of a struggle. Also, her passport’s missing, so she could have gone anywhere – by ferry, if not by plane. They said they’re happy to hand it over to a private investigator for now, which is why Dr MacDonald contacted us. There’s an agreement that we must tell the police if we turn up anything serious. And they said they’ll have to intervene if we haven’t found her by Friday evening.’

      ‘We need to get a move on then,’ I say. ‘What else is in that folder?’

      ‘Not much – it’s waiting to be filled. Oh – I’m meant to give you this.’ He hands me a fake ID badge, with my name beneath my photo and a company name.

      ‘Who are Prodigal Investigations?’ I ask.

      ‘That’s our undercover employer, while we’re working this case. It avoids awkward questions about the Guild. The story goes that we’ve been recruited by a PI agency that specialises in hiring promising young people. It’s just to show to anyone who asks too many questions.’

      ‘Fair enough,’ I say and stash the badge in the outside pocket of my backpack.

      He skims through his notes. ‘What I found out from my tête-à-tête with Dr MacDonald was that she’s approaching retirement, and that she’s from an old Scottish clan who own lots of land. They even have an island! It’s called the Isle of Fairhaven. She’s planning on going to live there when she retires from the gallery.’ He puts on a pretty convincing old lady’s voice – complete with Scottish accent – and says, ‘I’m going to pass the autumn of my years on the Isle of Fairhaven.’

      I laugh. ‘Is that what she sounds like?’

      ‘It is, and that’s what she said, verbatim, when I interviewed her yesterday.’ He slips back into Scots mode. ‘She’s such a dear, wee little thing.’ If I’m honest, part of me is a bit uncomfortable about his mockery of Dr MacDonald (I’ve been on the receiving end of too much teasing myself) but I can’t help laughing again – he’s too funny.

      ‘Her own island,’ I murmur. I picture the tiny plot of land in the Serpentine, to which I’ve rowed from time to time, and wonder how big the MacDonald clan’s isle might be.

      ‘So where do you think we should start?’ he asks me.

      I’m flattered that Arthur thinks enough of me to ask my opinion, when he’s clearly the more experienced agent. I do a mental run-through of important early procedures, from a book I’ve read five times: Complete Crime Scene Investigation Handbook. It tells you that one of the first things is to think of the obvious, and so I say, ‘Have you been to Sheila’s home yet, to search for clues?’

      He shakes his head. ‘No. I haven’t really started yet.’

      I draw Sheila’s photo close for a good look. She’s probably in her early fifties, dressed in a trouser suit, with one hand in her trouser pocket. With her glossy, blonde, shoulder-length hair, she has a vintage-film-star quality, like Greta Garbo or Rita Hayworth.

      ‘When was Sheila last seen?’ I ask him. ‘I mean, I know it was Friday night, but what time and where?’

      ‘At work. She got her coat at five thirty and said goodbye to all the staff. Apparently, she prides herself on knowing the names of all her colleagues, on both the art history and art maintenance sides.’

      I like the sound of Sheila.

      ‘And did anyone witness her leaving through the main entrance?’

      He consults his notebook. ‘No. The person on reception was busy with a tour group, so nobody actually saw her go.’

      ‘So she might have been kidnapped directly from the gallery.’

      ‘Or she might even still be there,’ he suggests. ‘Either hiding, for some reason, or tied up by an assailant.’

      This sounds unlikely to me. ‘Surely someone would have come across her by now, if she was being kept hostage in the building.’

      ‘A good investigator doesn’t rule anything out,’ he says.

      ‘True. So we need to check the security cameras to make sure she did leave, and see what time it was.’

      ‘Good idea.’

      I Change Channel and summon up a view of the National Gallery, with its roof removed, as if I’m floating above it. If I was Sheila Smith and I wanted to hide here, where would I go? And if I was her assailant, where would I put her, alive or dead?

      It takes me a moment to realise Arthur is speaking to me. ‘Hello? Earth calling Agatha …’

      ‘Sorry!’

      ‘Where did you go?’ he asks.

      I blush. ‘I just switched off this room inside my head and shone a light inside the gallery building.’

      Most of the time, people look at me politely or with mild concern when I explain my Change-Channel mechanism. Not Arthur, though. ‘Oh – I do that!’ he says enthusiastically. ‘I call it Auto-Focusing!’

      ‘Changing Channel!’ I say. I catch his eye and we laugh.

      ‘I guess the Guild attracts a certain brand of weirdo,’ he says.

      ‘I prefer “maverick”,’ I say. ‘You know – someone who’s happy to do things their own way.’

      He grins. ‘OK. Maverick it is. Let the investigation begin!’