2001, New York

Sébastien Cabot kept his eyes firmly clenched shut, not daring to get his first glimpse of the underworld and the Devil’s workings. Through his closed lids he could sense it was a dark place. His ears picked out sounds he’d never heard before, soft beeps and hums that could only be the devices of evil stirring, ready to tear his mortal soul apart.

‘And who’s this?’ a female voice echoed. He sensed they were standing in some cave, perhaps on a ledge that overlooked an infinite cavern filled with a squirming sea of tormented souls below, burning in agony, prodded, stabbed and tortured by demons wandering among them.

‘Cabot.’ He recognized Lady Rebecca’s voice in reply.

‘Cabot? Like – like in the message? The same guy?’

‘Affirmative. He is here to help.’

Cabot slowly opened his clenched-shut eyes. He first saw his sandalled feet on a hard pockmarked and stained stone floor. And as he looked up he saw Lady Rebecca and three other strangely dressed people staring at him with curiosity. No demons. No fire. No tormented souls.

One of them stepped forward. A young woman. She had long frizzy hair and pale freckled skin. On her face were two ovals of glass that glinted reflection from a bright bar of light above him that fizzed and flickered slightly.

‘Hey, pleased to meet you,’ she said, extending a hand towards him. ‘I’m Maddy.’

Cabot’s dry mouth opened and closed without producing anything. Finally, he managed to say something. ‘This … this place? ’Tis not … Hell?’

The frizzy-haired girl shrugged and smiled in a friendly way. ‘Guess it’s a matter of opinion really.’

Some time later – Cabot, still lost in a state of numb shock, had no idea how long: perhaps an hour, a day, perhaps only a few minutes later – he found himself and these curious strangers sitting around a long wooden table on padded chairs. He held a cup full of a warm and bitter brown drink. The other girl in this place, dark-skinned like a Turk and wearing black clothes splashed with a lurid orange and pink design of some sort – she’d been introduced as Sal – had told him the drink was called Koff-eeee as she’d pressed it into his hands.

‘… has a section in it that is marked by four corner markers,’ Lady Rebecca was saying. ‘In these margin illuminations. Here, here, here and here.’

The man with them – called Adam – hunkered over the table beside Lady Rebecca and examined the Treyarch Confession more closely.

‘My God, I think she may be right!’ he said. ‘They’ve got to be grille markers.’

Maddy joined them slumped over the table. She pushed her glasses up her nose. ‘Show me.’

‘See?’ said Adam. ‘Like corners, embedded subtly into the illustration’s pattern?’

‘Yeah – oh man, yeah, I see it!’

Adam looked at them. ‘Basically this is the blueprint … instructions on how to make a cardan grille to decode that,’ he said, pointing to the small wooden box containing the Grail.

‘It is this passage of text,’ continued Becks, spreading her hands across the Treyarch, ‘that is being indicated. But it is in a language we do not have data for. An extinct form of Gaelic. Cabot,’ she said, pointing at him, ‘has knowledge of this language.’

All eyes suddenly rested on him. He put down the mug of hot liquid on the table and spread his hands apologetically. ‘I … uh … I know but a little of it,’ he said. ‘I served the order alongside another Templar, Irish, a man who came from Dún Garbháin.’

Maddy cocked a finger, inviting him to lean over. ‘Well, come on and take a closer look, Mr Cabot. See what you can figure out.’

Cabot pulled himself up out of his chair and joined them over the document, rolled out and spread flat along the table and almost as long. A steadily burning light in a small wire cage dangled from the arched brick roof just above them. He wondered what made it glow so steadily. It was certainly no flame.

He turned his attention to the elaborate curls and flourishes of handwriting before him. By contrast to the feeble flickering candlelight the priory’s monks worked to after dark, this steady light let him see as if the table was standing outside in a field in the middle of a bright summer’s day.

‘This,’ he began, moving a leathery old finger along the first lines of text, ‘this I believe ’tis a form of Irish Gaelic.’ His finger traced the words, his lips moving in silence for a while.

‘I tell ye, ’tis hard to read, but … I think this first line is a prayer of silence. Be thy true a servant, or perchance that reads … to help?’ Cabot growled with frustration. ‘Seek ye not in … matters of truth of … or ’tis some other meaning. Take not matters of light … Achh! My Gaelic is too poor to read this.’

‘What do you think that’s supposed to mean?’ asked Sal. ‘Matters of truth?’

Maddy shrugged. ‘Just a bunch of weird voodoo crock.’ She looked at Cabot. ‘Is there some hidden meaning in there that we’re supposed to get? Like … is it cryptic or something?’

He shook his head. ‘’Tis as close as I can understand it. The rest is beyond me.’

They stared at the swirls and strokes of letters, a meaningless jumble of accented and contorted Latin letters.

‘Maybe it’s not actually meant to mean anything,’ said Adam. The others looked at him. ‘Maybe it’s not the words themselves that are the clue, but how they’re written? Cabot?’

The old man shrugged. He studied it in silence for a while. ‘My knowledge is poor, ye understand? ’Tis been a long while …’ He hesitated a moment.

‘What?’ asked Maddy. ‘What is it?’

He shook his head. ‘’Tis poorly written. This man, Treyarch, was clearly no scribe.’

‘What do you mean?’

Cabot pointed to one of the words in the sentence he’d loosely translated. ‘This letter is wrong. ’Tis written upside down.’

Adam hunched closer to it, his nose almost touching the yellowed parchment. ‘I wonder …’ he whispered to himself.

‘Wonder what? Adam?’

He looked up at them. ‘You got a decent digital camera?’

‘I’ve got my iPhone,’ replied Maddy.

‘What’s that?’

Of course, she smiled. 2001 … they’re still a twinkle in the eye of some Apple designer.

‘It’s just my cell, it’s got a built-in camera.’ She went over to the computer desk and returned with it a moment later.

‘Get a decent image of all of the text between these corner markers,’ said Adam.

She climbed up on to one of the armchairs to get a good bird’s-eye view of the scroll, then snapped several images. ‘What now?’

‘Photoshop,’ said Adam, pointing back towards the bank of computer monitors.

A minute later and Maddy had downloaded the four images she’d taken of the Confession on to one of the computers and they were looking at them within the image-editing software. Cabot’s eyes were comically round with wonder as he stared at the dozen glowing monitors.

‘So,’ said Adam, clicking on a dropdown menu. ‘I’m going to lighten these images up a little.’ He selected the clearest of the four images, and tweaked its brightness. The rich yellow of the parchment became a lighter vanilla, and the black ink became a deep blue.

‘Thing is,’ continued Adam, ‘when using a grille, you place it down on the blank parchment with the windows already cut out, and then you write each letter of your message on the little windows of parchment you can see. Then, when you’re done, you let the ink dry first before removing the grille, so you don’t smudge it. That would give the game away, right? Only certain letters being smudged?’

The others nodded. Made sense.

‘So, what you have then is a page of isolated letters … you write the rest of some meaningless or innocent-sounding message that incorporates those letters.’

He clicked on a menu and pulled another dropdown of editing options. ‘But quite often, in between these two stages, you might be writing with a different pot of ink.’

‘It’s the same colour,’ said Maddy, pointing at the image on-screen. ‘It’s black … well, dark blue now you’ve lightened it.’

‘Every pot of ink is slightly different. You made your own ink back then.’

Cabot nodded. ‘This is right.’

‘It’s home-made ink, not factory made. Every time you make it, it’s ever so slightly different. To our eyes, yes, it’s all black ink, but in Photoshop, just one variation of the RGB value …’

‘RGB?’

‘Red, Green, Blue – essentially, tone … hue,’ said Adam, ‘and we can separate it out. Exaggerate it enough to see.’ Adam zoomed in close on the writing, then selected another menu option producing a slide bar. The mouse cursor dragged the slide marker and moved it slowly along the horizontal bar. The image started shifting tone, the paper easing from vanilla to amber to pink. And the ink sliding from a deep blue to a deep green to a deep ochre.

‘Oh my God,’ whispered Maddy.

The upside-down letter that Cabot had identified was a slightly yellower ochre than the rest.

‘Zoom out,’ she said quickly. Adam did so, pulling out until the whole of the captured section of text was on the screen. Among the page, several hundred characters stood out distinctly from the rest – as distinct as minstrels at a banquet.