I’ve been sitting in front of Sadie’s portrait in the London Portrait Gallery for about two hours. I can’t drag myself away. She’s gazing out at the gallery, her brow clear, her eyes a velvety dark green, like the most beautiful goddess you’ve ever seen. Cecil Malory’s use of light on her skin is unmatched in its artistry. I know, because I heard an art teacher telling her class half an hour ago. Then they all went up to see if they could spot the little miniature portrait in the necklace.

I must have seen a hundred people coming and looking at her. Sighing with pleasure. Smiling at one another. Or just sitting down and gazing.

“Isn’t she lovely?” A dark-haired woman in a mac smiles at me and sits down beside me on the bench. “This is my favorite portrait in the whole gallery.”

“Me too.” I nod.

“I wonder what she’s thinking?” the woman muses.

“I think she’s in love.” I look yet again at Sadie’s glowing eyes, the flush in her cheek. “And I think she’s really, really happy.”

“You’re probably right.”

For a moment we’re both quiet, just drinking her in.

“She does you good, doesn’t she?” says the woman. “I often come and look at her in my lunchtimes. Just to cheer myself up. I’ve got a poster of her at home too. My daughter bought it for me. But you can’t beat the real thing, can you?”

There’s a sudden lump in my throat, but I manage to smile back. “No. You can’t beat the real thing.”

As I’m speaking, a Japanese family approaches the painting. I can see the mother pointing out the necklace to her daughter. They both sigh happily, then adopt identical poses, arms folded, heads tilted, and just gaze at her.

Sadie’s adored by all these people. Tens, hundreds, thousands. And she has no idea.

I’ve called for her until I’m hoarse, over and over, out the window, up and down the street. But she doesn’t hear. Or she doesn’t want to hear. Abruptly, I stand up and consult my watch; I have to go, anyway. It’s five o’clock. Time for my appointment with Malcolm Gledhill, the collections manager.

I make my way back to the foyer, give my name to the receptionist, and wait among a swarming crowd of French schoolchildren until a voice from behind says, “Miss Lington?” I turn to see a man in a purple shirt, with a chestnut beard and tufts of hair growing out of his ears, beaming at me with twinkly eyes. He looks like Father Christmas before he grew old, and I can’t help warming to him instantly.

“Hi. Yes, I’m Lara Lington.”

“Malcolm Gledhill. Come this way.” He leads me through a hidden door behind the reception desk, up some stairs, and into a corner office overlooking the Thames. Postcards and reproductions of paintings are everywhere, stuck up on the walls and propped against books and adorning his massive computer.

“So.” He hands me a cup of tea and sits down. “You’re here to see me about Girl with a Necklace?” He eyes me warily. “I wasn’t sure from your message quite what the issue was. But it’s clearly… pressing?”

OK, perhaps my message was a bit extreme. I didn’t want to have to tell the whole story to some nameless receptionist, so I simply said it was to do with Girl with a Necklace and a matter of life and death, state urgency, and national security.

Well. In the art world, it probably is all those things.

“It is quite pressing.” I nod. “And the first thing I want to say is, she wasn’t just ‘a girl.’ She was my great-aunt. Look.”

I reach into my bag and produce my photograph of Sadie at the nursing home, wearing the necklace.

“Look at the necklace,” I add, as I hand it over.

I knew I liked this Malcolm Gledhill guy. He reacts in a totally satisfactory manner. His eyes bulge. His cheeks turn pink with excitement. He looks up sharply at me, then back at the photo. He peers at the necklace around Sadie’s neck. Then he gives a harrumphing cough as though he’s concerned he’s given too much away.

“Are you saying,” he says at last, “that this lady here is the ‘Mabel’ in the painting?”

I really have to knock this Mabel thing on the head.

“She wasn’t called Mabel. She hated the name Mabel. She was called Sadie. Sadie Lancaster. She lived in Archbury and she was Stephen Nettleton’s lover. She was the reason he was sent to France.”

There’s silence, apart from Malcolm Gledhill breathing out, his cheeks two deflating balloons of air.

“Do you have any evidence that this is the case?” he says at last. “Any documents? Any old photographs?”