Tom listened to all this without interrupting and she realised when she had finished that it had been strangely calming to talk things through, even if she barely knew him. There had been so much going on, so many thoughts tripping over each other inside her head, that it had been surprisingly cathartic to lay all the different elements together end to end.

‘Somehow, it’s all linked,’ he said slowly when she had finished. ‘The murders, Caravaggio, the symbol…we just need to find out how.’

‘Is that all?’ she said with a bitter laugh.

‘Sometimes you just need to know who to ask.’

‘And you do?’ she asked in a sceptical tone.

‘I know someone who might be able to help.’ He nodded.

‘Someone we can trust?’

Tom took a deep breath, then blew out his cheeks.

‘More or less.’

‘What sort of an answer’s that?’ she snorted.

‘The sort of answer you get when you’re out of better ideas.’

There was a pause. Then with a resigned shrug she started the engine.

‘Where to?’

Fontana di Trevi, Rome 19th March-8.03 a.m.

Allegra heard the fountain before she saw it, a delirious, ecstatic roar of water that crashed and foamed over gnarled travertine rocks and carved foliage, tumbling in a joyful cascade into the open embrace of the wide basin below. This was no accident, Allegra knew, the Trevi having been deliberately positioned so that, no matter what route was taken, it could only be partially seen as it was approached, the anticipation building as the sound got louder until the monument finally revealed itself.

Despite the relatively early hour, the tourists were already out in force, some seated like an eager audience on the steps that encircled the basin’s low stage, others facing the opposite direction and flinging coins over their shoulders in the hope of securing their return to the Eternal City. Oblivious to their catcalling and the popcorn burst of camera flashes, the statues ranged above them silently acted out an allegorical representation of the taming of the waters. Centre stage loomed Neptune’s brooding figure, his chariot frozen in flight, winged horses rearing dramatically out of the water and threatening to take the entire structure with them.

‘Was there a Trevi family?’ Tom asked as they paused briefly in front of it.

‘Trevi comes from Tre Via, the three streets that meet here,’ she corrected him in a curt voice. ‘Are we here for a history lesson or to actually see someone?’

‘That depends,’ he said with a shrug.

‘On what?’

‘On whether you can keep a secret.’

She gave a dismissive laugh.

‘How old are you, ten?’

Tom turned to face her, face set firm.

‘You can’t tell anyone about what you see.’

‘Oh come on,’ she snorted impatiently.

‘Yes or no?’ he insisted.

There was a pause. Then she gave a grudging nod.

‘Yes, fine, whatever.’

‘No crossed fingers?’

‘What?’ she exploded. ‘If this is some sort of…’

‘I’m only joking.’ He grinned. ‘Come on. It’s this way.’

He led her round to the right to the Vicolo Scavolino where a small doorway had been set into the side wall of the building directly behind the fountain. A flock of pigeons rendered fat and tame by years of overfeeding, barely stirred as they waded through them.

‘Here?’ she asked with a frown, glancing up at the carved papal escutcheon suspended over the entrance.

‘Here.’ He nodded, knocking sharply against the door’s weather-worn surface.

A few moments later it opened to reveal a young Chinese man dressed in black, his hair standing off his head as if he had been electrocuted. From the way he was awkwardly holding one hand behind his back, Allegra guessed that he was clutching a gun.

‘I’m here to see Johnny,’ Tom announced. ‘Tell him it’s Felix.’

The man gave them a cursory look, then shut the door again.

‘Felix?’ Allegra shot him a questioning look.

‘It’s a name people used to know me by when I was still in the game,’ he explained. ‘I try not to use it any more, but it’s how a lot of people still know me.’

‘The game?’ She gave a hollow laugh. ‘Is that a word people like you use to make you feel better about breaking the law?’

The door reopened before Tom had a chance to answer, the man ushering them inside and then marching them along a low passageway, through a second door and then up a shallow flight of steps into a narrow room, with a stone staircase leading both up and down.

‘Where are we?’ Allegra hissed.

‘Listen,’ Tom replied.

She nodded, suddenly realising that the dull ringing in her ears was no longer the angry echo of the shot that had killed Gambetta but the muffled roar of water through the thick walls.

‘We’re behind the fountain,’ she breathed.

‘The Trevi was pretty much tacked on to the façade of the Palazzo Poli when they built it,’ Tom explained as the man ordered them up the stairs with a grunt. ‘This space was bricked off as a maintenance shaft, to provide access to the roof and the plumbing in the basement. Johnny cut a deal with the mayor to rent the attic.’

‘You’re kidding, right?’

‘Why? How else do you think he paid for his re-election campaign?’

They climbed to the first floor, then to the next, the fountain’s low rumble slowly fading, until it was little more than a distant hum. In its place, however, Allegra was increasingly aware of a whirring, rhythmical clattering noise. She glanced at Tom for an explanation, but he said nothing, his expression suggesting that he was rather enjoying her confusion.

Another man was waiting to greet them on the second-floor landing, a machine gun slung across his oversized Lakers shirt, in place of the rather less threatening Norinco Type 77 handgun that their escort was sporting. The higher they climbed, the more lethal the weaponry, it seemed.

The second man signalled at them to raise their arms and then quickly patted them down, confiscating Tom’s bag and Allegra’s gun and keys. Then he nodded at them to follow him to the foot of the next flight of stairs, where an armoured steel door and two more guards blocked their way. Unprompted, the door buzzed open.

Swapping a look, they made their way upstairs.

19th March-8.12 a.m.

The staircase led to a long, narrow attic room that seemed to run the width of the entire building. A line of squat windows squinted down on to the square below, their view obscured in places by the fountain’s massive stone pediment. And running down the centre of the room, hissing and rattling like an old steam engine under the low ceiling, was a huge printing press.

‘The sound of the fountain masks the noise of the machine,’ Tom called to her over the press’s raucous clatter as she approached it. ‘It’s actually five separate processes, although the machines have been laid out end to end. A simultan machine to print the background colours and patterns. An intaglio machine for the major design elements. A letterpress for the serial numbers. An offset press for the overcoating. And obviously a guillotine right at the end to cut the sheets to size.’

Allegra stepped closer to the press, trying to catch what was coming off the machine’s whirling drum, then looked back to Tom in shock.

‘Money?’

‘Euros.’ He nodded. ‘Johnny runs one of the world’s biggest counterfeiting operations outside of China. He used to print dollars, but no one wants them any more.’

‘Johnny who?’ she asked, looking back along the room and noticing the small army of people in blue overalls tending silently to the press.

‘Johnny Li. His father is Li Kai-Fu. Runs one of the most powerful Triad gangs in Hong Kong,’ Tom explained in a low voice. ‘A couple of years ago he posted his five sons around the world, via Cambridge, to help grow the family business. Johnny’s here, Paul’s in San Francisco, Ringo’s in Buenos Aires…’