Hutton's account opens in a pub called the Black Dog public house, where the narrator falls into conversation with a poor thin-gut fellow - a stranger who appears unwell, perhaps half-starved. The stranger tells the narrator the story behind the pub's name. During the reign of Henry III, he says, a famine struck England. Conditions inside Newgate Prison, never good, became unspeakable. The starving inmates had already turned to cannibalism, killing and eating those too weak to resist. One day a scholar was committed to the gaol - a man with the reputation of being a sorcerer, who had done much hurt to the king's subjects with his charms and devilish witchcraft. The starving prisoners fell upon him almost as soon as he arrived. They killed him. They ate him. Shortly after the murder, those guilty of it began to see something in the corridors of the prison: a monstrous black dog walking up and down. They became convinced it was the sorcerer's spirit, returned in the shape of a beast to take revenge. One by one the murderers were killed and consumed by the apparition, until the last few survivors, mad with terror, broke out of the gaol and fled. The dog followed them. It hunted them through London and across England, the stranger said, killing them wherever they tried to hide, until every man who had eaten the sorcerer was dead.

There is a careful ambiguity at the heart of Hutton's pamphlet. The Thin-gut fellow telling the story has obviously suffered. Towards the end of the conversation, he claims that the supernatural tale of the dog is untrue. The only Black Dog he knows of, he says, is a great black stone standing in the dungeon called Limbo, the place where condemned prisoners are kept after sentencing - against which felons have dashed out their brains in their distress. It is the kind of confession that suggests the stranger himself may have been one of the prisoners, that he may have known the dungeon called Limbo from the inside, that he may even have heard the steps of something terrible coming for him in the dark. Hutton presents the whole thing as a morality tale - a warning against the low behaviour and base living of inmates, urging reform of the appalling conditions in the prison. He dedicated the pamphlet to Lord Chief Justice Sir John Popham, who was instrumental in deciding the fate of prisoners awaiting trial. Some scholars believe the dedication helped secure Hutton's release. He was, after all, writing the work from inside Newgate itself.

The Black Dog of Newgate fits into a much older tradition. Across England, ghost stories about spectral hounds have been recorded since at least the Anglo-Saxon period. Different counties name them differently: Black Shuck in East Anglia, Padfoot in West Yorkshire, the Barghest around the Pennines, the Gytrash on the moors of the Brontë sisters. They are usually massive, jet-black, often with glowing red eyes. They appear at crossroads, on lonely roads at night, in churchyards, near places of execution. They are almost always ill omens. To see one was to know that someone close to you was about to die - or that you yourself were marked. The Newgate version takes this archetype and roots it in a specific London horror: the famine, the cannibalism, the closed-in prison full of guilty men. There is no archaeological or chronicle evidence that the underlying cannibalism actually happened during the reign of Henry III - the story may well be folklore inserted into a historical frame. But the Black Dog's hold on the imagination of London was real. By Hutton's time the tale was already considered ancient. People crossed themselves when they passed the prison after dark.

There is a quieter sequel to the story that survives in a different form. Just behind the site of the old Newgate Prison, beyond a high brick wall, stood Amen Court - a small enclosed courtyard of houses belonging to the canons of St Paul's Cathedral. The wall separated the prison precinct from the cathedral precinct, the worst place in London from one of its most respectable. After Newgate was demolished in 1902, the wall remained. And reports persisted into the 20th century of an amorphous Black Shape that crept along the top of that wall at night, never quite resolving into a recognisable form. Witnesses described something like a deep shadow that moved against the brick, sometimes glimpsed from the courtyard below, sometimes from the alley behind. No one ever got a clear look at it. It was never a dog exactly, never a man, never anything that could be properly named. Some who recorded the sightings believed it was the same haunting as Hutton's - the medieval black dog moved from the demolished gaol to the only piece of the wall that had outlived its original purpose.

Newgate Prison was rebuilt in stone in the 18th century, partially destroyed during the Gordon Riots in 1780, rebuilt again, and finally demolished in 1902. The Central Criminal Court of England and Wales, more commonly called the Old Bailey, was built on the site. The dome of the Old Bailey is topped with a bronze figure of Justice carrying a sword in one hand and a set of scales in the other, completed in 1906 by F.W. Pomeroy. The court is still in session every day of the year except Sundays and bank holidays. The original Black Dog public house where Luke Hutton supposedly met his thin-gut stranger is long gone, but the pamphlet survives. A notable copy with its original 1596 woodcut frontispiece - showing a man cowering before a great black hound - is held at the Guildhall Library a few streets to the east. Amen Court still exists, although its wall has been rebuilt several times and the canons' houses are now mostly private offices. If anything still walks the wall at night, the people who work behind the cathedral keep what they have seen to themselves.