He discovers these facts: that Édouard Lefèvre had, until recently, been the least famous artist of the Académie Matisse. There are only two academics who specialize in his work, and neither of them knows any more than he does about The Girl You Left Behind.

A photograph and some written journals obtained by the Lefèvre family have turned up the fact that the painting hung in full view in the hotel known as Le Coq Rouge in St Péronne, a town occupied by Germans during the First World War. It disappeared without trace some time after Sophie Lefèvre was arrested.

And then there is a gap of some thirty years before the painting reappears, in the possession of one Louanne Baker, who kept it in her home in the US for thirty years until she moved to Spain, where she died, and David Halston bought it.

What happened to it between those dates? If it really was looted, where was it taken? What happened to Sophie Lefèvre, who seems to have simply vanished from history? The facts exist, like the dots in a join-the-dots puzzle but one in which the picture never becomes clear. There is more written about Sophie Lefèvre’s painting than there is about her.

During the Second World War, looted treasures were kept in secure vaults in Germany, underground, protected. These artworks, millions of them, had been targeted with military efficiency, aided by unscrupulous dealers and experts. This was not the random plunder of soldiers in battle: this looting was systematic, controlled, regulated and documented.

But there is little surviving documentation from the First World War, regarding looted property, especially in northern France. It means, Janey says, that this is something of a test case. She says it with some pride. For the truth is, this case is vital to their company. There are increasing numbers of organizations like theirs springing up, all sourcing provenance, listing works that relatives of the dead have spent decades trying to trace. Now there are no-win no-fee firms undercutting them, promising the earth to people who are willing to believe anything to get their beloved object back.

Sean reports that Liv’s lawyer has tried various legal means to get the case struck out. He claims that it falls beyond the statute of limitations, that the sale to David from Marianne Baker had been ‘innocent’. For a variety of complicated reasons, these have all failed. They are, says Sean, cheerfully, headed to court. ‘Looks like next week. We have Justice Berger. He’s only ever found for the claimant in these cases. Looking good!’

‘Great,’ says Paul.

There is an A4 photocopy of The Girl You Left Behind pinned up in his office, among other paintings missing or subject to restitution requests. Paul looks up periodically and wishes that every time he did so Liv Halston wasn’t looking back at him. Paul switches his attention to the papers in front of him. ‘This image is such as one would not expect to find in a humble provincial hotel,’ the Kommandant writes to his wife at one point. ‘In truth I cannot take my eyes from it.’

It? Paul wonders. Or her?

Several miles away, Liv is also working. She rises at seven, pulls on her running shoes and heads off, sprinting alongside the river, music in her ears, her heartbeat thumping along with her footsteps. She gets home after Mo leaves for work, showers, makes herself breakfast, drops a tea in with Fran, but now she leaves the Glass House, spending her days in specialist art libraries, in the fuggy archives of galleries, on the Internet, chasing leads. She is in daily contact with Henry, popping in whenever he asks to hold a conference, explaining the importance of French legal testimony, the difficulty of finding expert witnesses. ‘So basically,’ she says, ‘you want me to come up with concrete evidence on a painting about which nothing has been recorded of a woman who doesn’t seem to exist.’

Henry smiles nervously at her. He does this a lot.

She lives and breathes the painting. She is blind to the approach of Christmas, her father’s plaintive calls. She cannot see beyond her determination that Paul should not take it. Henry has given her all the disclosure files from the other side – copies of letters between Sophie and her husband, references to the painting and the little town where they lived.

She reads through hundreds of academic and political papers, newspaper reports about restitution: about families destroyed in Dachau, their surviving grandchildren borrowing money to recover a Titian; a Polish family, whose only surviving member died happy two months after the return of her father’s little Rodin sculpture. Nearly all these articles are written from the point of view of the claimant, the family who lost everything and found the grandmother’s painting against the odds. The reader is invited to rejoice with them when they win it back. The word ‘injustice’ appears in almost every paragraph. The articles rarely offer the opinion of the person who had bought it in good faith and lost it.

And everywhere she goes she detects Paul’s footprints, as if she is asking the wrong questions, looking in the wrong places, as if she is simply processing information that he has already acquired.

She stands up and stretches, walking around the study. She has moved The Girl You Left Behind on to a bookshelf while she works, as if she might give her inspiration. She finds herself looking at her all the time now, as if she is conscious that their time together may be limited. And the court date draws ever closer, always there, like the drumbeat of a distant battle. Give me the answers, Sophie. At the bloody least, give me a clue.

‘Hey.’

Mo appears at the door, eating a pot of yoghurt. Six weeks on, she is still living in the Glass House. Liv is grateful for her presence. She stretches and checks her watch. ‘Is it three o’clock already? God. I’ve got almost nowhere today.’

‘You might want to take a look at this.’ Mo pulls a copy of the London evening paper from under her arm and hands it over. ‘Page three.’

Liv opens it.

Award-winning Architect’s Widow In Million-pound Battle For Nazi-looted Art, the headline says. Underneath is a half-page picture of David and her at a charity event several years previously. She is wearing an electric blue dress and is holding up a champagne glass, as if toasting the camera. Nearby is a small inset picture of The Girl You Left Behind with a caption: ‘Impressionist painting worth millions was “stolen by German”.’

‘Nice dress,’ says Mo.

The blood drains from Liv’s face. She does not recognize the smiling partygoer in the picture, a woman from a different life. ‘Oh, my God …’ She feels as if someone has thrown open the doors of her house, her bedroom.