A tentative knock sounded from the front of the house; his heart leaped. It was shameful, but he didn’t care. She’d come back. He jumped to his feet and ran to the door, gathering the dressing gown around his thin waist. Without even bothering with the peephole, he unlatched the door and pulled it open.

Two men stood on the doorstep. Both wore heavy black coats with the hoods raised against the cold. They pushed inside before he could react. Jerome stumbled back and fell against the wall. The shorter of the two men unzipped his jacket and rested his hand inside the lapel; the other man pulled back his hood to reveal a craggy face with a patrician crest of white hair, and coal-black eyes that seemed to bore into Jerome’s soul.

Jerome stared. ‘You.’

He had only met him once, thirty years ago: a Spanish priest from an obscure office of the Vatican, visiting a promising young researcher who had just begun to make a name for himself. Even then, menace surrounded him. He had spent half an hour asking about Jerome’s work – always stiffly formal, but lethal, poised like a fencer probing his opponent’s guard. At the end he had said, ‘There are many undiscovered books in this world. Some are treasures undeservedly lost; others vanished for a reason and should remain forgotten. If you ever find one of these latter books, you must tell me at once.’

In the years afterwards, Jerome had occasionally seen photographs of the priest – at first only in Church bulletins, then in newspapers and finally even on television. In the whispered gossip of his order he heard rumours about the methods the priest had used in his rise to power, and believed them.

And now he was standing in Jerome’s living room, beside a squat thug with a broken nose and a livid scar across his chin. A cardinal’s jewelled ring gleamed on his finger. He looked around the dishevelled room, at the half-empty coffee mugs clustered around the chair.

‘You have had visitors today?’

‘Only memories.’

Behind Nevado, the thug pulled his arm out of his coat. A black pistol had appeared in his hand. He squinted down the barrel as he pulled back the slide and snapped it home. The sound made Jerome wince.

‘Sometimes memories come to life.’ Nevado moved forward; Jerome cringed, pressing his bony shoulders against the wall. ‘You, Brother, have good reason to fear them.’

Jerome looked into those pitiless eyes. He didn’t even try to hold their gaze. His spirit had been broken long ago. He couldn’t resist: they would find out everything.

‘She came here,’ said Nevado. ‘Emily Sutherland – your little Héloïse. Did she bring you a book?’

‘No one came here.’

Jerome’s head snapped against the wall as Nevado struck him, a stinging blow. Blood dribbled from his lip where the cardinal’s ring had cut him.

‘Liar. She was here. Did she bring her new boyfriend to flaunt him? To taunt you? Did she offer you her body again if you would help them?’

Jerome’s dressing gown sagged open. His naked body seemed to shrivel under Nevado’s glare. He imagined Nevado’s hands on Emily’s throat, that cold smile never wavering.

There was only one way to protect her. Jerome launched himself forward, pushing off the wall as he lunged past Nevado for the pistol. He knew he wouldn’t make it. The gun came up and fired three times into Jerome’s chest. The first bullet went straight through his heart. He collapsed on the floor, his blood pumping into the carpet.

‘Idiot,’ hissed Nevado. ‘We needed him to talk.’ He gazed around the room. So many books, so much chaos. It would take hours to search the house. He had an audience in Rome in three hours: people would talk if he missed it. Gossip didn’t matter to him, but if anyone looked into where he’d been there might be trouble. He couldn’t risk being discovered here.

But Nevado had built his career on seeing what other men could not. He stood very still in the centre of the room and slowly scanned it, dismantling it with his eyes. Ugo, the guard, waited behind him.

He looked through an open door to the study beyond. He saw a desk whose jumble of books and papers had been pushed back to clear an open space. A magnifier, a UV penlight, a foam cushion and a pair of white gloves filled the space.

In an instant, Nevado had crossed into the study and was examining the desk. Ugo came up behind him, surprised by how quickly the old man moved.

It didn’t take Nevado long to find everything he needed. Crumbs of worn leather littered the cushion, and a book beside it was weighted open to a page showing the queen of wild men. The notepad beside it displayed the list Jerome had made just before he died.

Nevado read over it.

Armand, Comte de Lorraine (Strasbourg??)

A shiver ran down his spine. They’d found it. His life’s work, now almost complete.

He turned to Ugo.

‘You go to Strasbourg. I will meet you there as soon as I can. Find the American and his friend, and find the book they have. That is all that matters.’

He reached in his coat and pulled out a folded sheet of paper.

‘If you find the book, tell me at once if the first page is the same as this. You understand?’

Ugo nodded. He took the paper – but Nevado had not let go. The black eyes locked on his.

‘If anything happens, if you are arrested or compromised, you destroy this paper immediately. No one can be allowed to see it. If you fail me in this, your wife, your children and all your family will suffer torments even you cannot imagine.’

His gloved fingers released the paper. Ugo stumbled back a step.

Almost to himself, Nevado murmured, ‘They have no idea what they have found.’

Strasbourg, France

Nick had never seen Strasbourg before. If he’d had an idea of how it would look, it probably involved great blocks of European concrete filled with parliaments, courts and commissions. Instead, he felt he’d stepped back a thousand years. The centre of the town was built on an island, the river a natural moat. Half-timbered houses hung over the narrow streets and alleys, funnelling the freezing wind so that it whipped snow in their faces. Many houses had fanciful creatures carved into their beams: grotesque faces sticking their tongues out at him in mockery.

A tram whistled past. Nick stuck out an arm to hold back Emily, who had been about to step out into the street.

‘Thanks.’ She gave him a sheepish smile. ‘I should have slept more on the train. I’m a wreck.’

Nick looked at her. She had piled her hair under her beret and turned up the collar on her coat. Her cheeks had flushed pink, and her eyes were bright in the cold. ‘You look pretty good for a wreck.’

Again, Emily seemed to flinch from the compliment. This time the smile was purely defensive. ‘I’ll feel better once I’ve had a shower and a hot meal.’

‘After we’ve been to the archives.’

They reached the cathedral, which dominated the heart of the city. Even with his mind on Gillian, Nick had to admire it. The facade was a vertiginous tangle of Gothic tracery: spires and pinnacles, a rose window, peaked arches and statues. A single tower stretched high above it, the pink sandstone spun to a lacy thinness that seemed incapable of supporting such a height.

Emily followed Nick’s gaze up the tower. ‘It’s almost exactly the same age as the playing cards. If the Master ever came here, he’d have seen it just the same way as we do.’

‘I’m more interested in if Gillian saw it three weeks ago.’

They carried on around the square, past rows of shops offering ice creams and souvenirs. Nick imagined that in summer tourists would swarm like wasps around their sticky offerings, but on a wintry day in January there was nobody. Half-empty wire racks of postcards sat forlornly on the pavement where they had been pushed out by hopeful shopkeepers, draped in polythene shrouds to keep off the snow. The plastic whipped and crackled in the wind, scaring the pigeons who scavenged on the cobbles.