Three other cases he’d run of suspected arsenic poisoning had turned out to be accidental. The toxin occurs naturally in groundwater, particularly where fracking – high pressure geologic fracturing to extract oil and gas – has occurred.

In fact, throughout history, for every intentional victim of arsenic poisoning – like Francesco I de’ Medici, Grand Duke of Tuscany – there were many more accidental victims: Napoleon Bonaparte, possibly done in by the wallpaper of the rooms to which he’d been exiled on St Helena; Simón Bolívar (the water in South America); and the American ambassador to Italy in the 1950s (flaking paint in her residence). It was also possible that the madness of King George was due to the metalloid.

‘Can we see him?’ Sachs asked.

‘I’m afraid not. He’s unconscious. But a nurse will call you when he comes to.’

Rhyme noted and, for Rachel’s sake, appreciated the conjunction.

When, not if.

The doctor shook hands. ‘You believe someone actually did this intentionally?’

‘That’s right.’

‘Oh, my.’

His mobile rang and without a word he turned away to answer.

In October 1818 an attractive woman with an angular face and piercing eyes died at the age of thirty four in Spencer County, Indiana.

There is some debate as to what was the cause of Nancy Lincoln’s death – possibly tuberculosis or cancer but the general consensus is that she was a victim of milk sickness, which claimed thousands of lives in the nineteenth century. Although the actual cause can’t be pinpointed, one fact about Nancy’s death is well documented: her nine year old son, Abraham, the future president of the United States, helped his father build the woman’s coffin.

Milk sickness perplexed medical professionals for years, until it was finally discovered that the cause was tremetol, a highly toxic alcohol, which made its way into a cow’s milk after the animal had grazed on white snakeroot.

This plant is a nondescript, workaday herb that is hardly an aesthetic contribution to any garden, and accordingly Billy Haven didn’t enjoy the plant as a subject to sketch. But he loved its toxic properties.

When ingested, tremetol causes the victim to suffer excruciating abdominal pain, intense nausea and thirst, uncontrollable tremors and explosive vomiting.

Even a small dosage can result in death.

Head down, wearing a short brimmed brown fedora – very hipster – and long black raincoat, Billy was making his way through Central Park, the west side. In his gloved hand was a briefcase. He was walking south and had made a serious trek from Harlem, but he wanted to avoid the CCTV cameras in the subway, even if his appearance was different from what the Underground Man had worn during the prior attacks.

Yes, tremetol was his weapon but the pending attack wouldn’t involve tattooing, so he’d left his machine back at his workshop near Canal. Today the circumstances dictated a different means of poisoning. But one that could be just as satisfying.

Billy was enjoying a good mood. Oh, with the earlier attacks, he’d felt satisfaction, sure, buzzing the poison into the victims, getting the bloodline just right, angling the careful serifs of the Old English letters.

A Billy Mod …

But that was good in the same way you felt good doing your job or completing chores around the house.

What he was about to do now was a whole different level of good.

Billy slipped out of the park and examined the streets carefully, uptown and cross, noting no one looking at him with suspicion. No police on patrol. He continued his journey south toward his target.

Yes, this attack would be different.

For one thing, there was no message to send. He’d simply deliver the tremetol. No scars, no tats, no mods.

Also, he was not interested in killing the victim. That death would ultimately be detrimental to the Modification. No, he was going to wield the poison to debilitate.

Though it would be a very different life that his target would live in the future; perhaps the most disturbing symptoms of non lethal white snakeroot poisoning were delirium and dementia. The man he was going to poison in a few moments would stay alive but become a raving madman for a long, long time.

Billy nonetheless had one regret: that his victim would be incapable of feeling the searing, unbearable nausea and gut pain that white snakeroot’s toxin caused. Lincoln Rhyme was numb to sensation below his neck. The vomiting, tremors and other symptoms would be unpleasant but not as horrific as in a person who had a fully functioning nervous system.

Billy now turned west down a cross street and entered a brightly lit Chinese restaurant, which was filled with the smells of garlic and hot oil. He made his way to the restroom, where, in a stall, he lost the hat and overcoat and dressed in coveralls.

Outside once more – unnoticed by diners or staff, he observed – Billy walked across the street and into the service alley that would lead to the back of Rhyme’s apartment.

The cul de sac was pungent – smelled a bit like the Chinese restaurant, now that he thought about it – but relatively clean. The ground was ancient cobblestones and patches of asphalt, dotted with slush and ice. Several Dumpsters sat well ordered against brick walls. It seemed that several town houses, including Rhyme’s, and a larger apartment building backed onto this area.

Noting a video camera at the rear of Rhyme’s town house, he went about his faux business of checking electrical lines.

Ducking behind a Dumpster, as if searching for a troublesome bit of electrical wire conduit, Billy circumvented the camera and approached the door. He extracted the hypodermic that contained the snakeroot toxin from his toothbrush holder and slipped the syringe into his pocket.

Tremetol, a clear liquid, is an alcohol and would blend instantly with what Billy’s research had revealed was Rhyme’s favorite beverage – single malt scotch. It would also be tasteless.

Billy’s palms sweated. His heart thudded.

For all he knew there might be ten armed officers inside, meeting with Rhyme at the moment. The alarm wouldn’t be on, not during the day, but he could easily be spotted lacing the bottle.

And possibly shot on the spot.

But the Modification, naturally, involved risk. What important missions didn’t? So, get on with it. Billy pulled out his phone, a prepaid model, untraceable, and pressed in a number.

Almost immediately he heard, ‘Police and fire. What’s your emergency?’

‘A man with a gun in Central Park! He’s attacking a woman.’

‘Where are you, sir?’

‘He’s got a gun! I think he’s going to rape her!’

‘Yes, sir. Where are you? Where exactly?’

‘Central Park West, about … I don’t know. It’s … uhm, okay, in front of Three Fifty Central Park West.’

‘Is anyone hurt?’

‘I think so! Jesus! Please. Send somebody.’

‘Describe him.’

‘Dark skinned. Thirties.’

‘What’s your name–?’

Click .

It was sixty seconds later that he heard the sirens. He knew the 20th Precinct, located in Central Park, was nearby.

More sirens.

Dozens of squad cars, he guessed.

He waited until the sirens grew louder; they’d have to be drawing the attention of everyone in the town house. Gambling that no one could see the security monitor, Billy walked matter of factly to Rhyme’s back door. Paused again. He looked around. Nobody. He turned to the lock.

Later, the police might look at the security tape – if it was recorded at all – and see the intruder. But all they’d see would be a vague form, head down.

And by then it would be too late.

‘The hell is going on?’ Rhyme barked.

The criminalist and Mel Cooper were in the front hallway of the town house, the door open. Ron Pulaski joined them. They were peering out into the street, which was filled with police cars, two ESU vans and two ambulances.