He was beautiful. He was impossible. And nobody could keep him.

In Venice, as in the rest of Italy, seventeen is an unlucky number. Written in Roman numerals, it becomes XVII, which, when rearranged, spells vixi: “I have lived”, an expression traditionally associated with death. It was this number that some chose to attach to the so-called Mystery Boy, for Venetians can be remarkably superstitious when darkness falls. Yet, as far as anyone knew, he had caused no harm and certainly had brought death to no one.

The story was reported in Il Gazzettino, which, with a modicum of certainty, identified the Mystery Boy as Bertie Van Tahn, a young man of Dutch and Flemish descent. He was said to be the seventeen-year-old son of wealthy bankers who spent much of the year in a sixteenth-century palazzo near the Rio dei Mendicanti, which marks the boundary between the Venetian sestieri of Cannaregio and Castello. The newspaper, however, was somewhat perplexed by the reasons for his eccentric behaviour.

There was another rumour, too, that circulated about the Mystery Boy: a centuries-old tale passed down from one generation to the next, embellished with every telling until myth had become reality and the whole affair had acquired an air of the plainly ridiculous.

It concerned the infamous Comte de Saint Germain, an enigmatic eighteenth-century adventurer and polymath whose origins remain uncertain, and who was often referred to as The Man Who Never Dies. Official records placed his death in February 1784, in Eckernförde, Germany, yet he was alleged to have been seen at intervals ever since.

Quite why the Comte de Saint Germain, Master of the Ancient Wisdom, should reappear as a teenage boy in Venice seemed unfathomable. Nevertheless, a wave of hysteria proved sufficient to unsettle even the soundest of minds. Those inclined to believe this absurd theory maintained that the enigmatic count had discovered an anti-ageing elixir, the secret of which permitted him to live indefinitely.

The rumour gained further momentum through tales circulating in the corridors of Università Ca’ Foscari and Scuola Navale Militare Francesco Morosini. Those who claimed to have seen the boy described his eyes as spectral violet and spoke of a Maltese cross, rendered in periwinkle, which he wore about his neck.

The excitement soon provided gondoliers with a novel means of beguiling their tourists. Standing tall at the stern, a silhouette of effortless grace, one might add a little something to the usual account of Venice: it was no longer safe, they would explain, to venture out after dark.

One thing, however, seemed certain. While respectable Venetians slept in their beds, someone, or something, roamed the age-old flagstones, wandered beside the splendid canali and slipped across deserted bridges. A rational observer might have scoffed at such foolishness, but the evidence mounted almost daily to suggest that someone did, after all, appear in the dead of night.

It was a young girl who first claimed to have seen the figure who would come to be known among her enthusiastic contemporaries as the Comte de Castello. She had observed the young man standing at the apex of the elegant stone Ponte dei Greci, an appropriate symbolic gateway into Castello, beneath the shadow of the leaning bell tower of nearby San Giorgio dei Greci.

The boy had stood for a long time, staring down into the dark waters of the Rio di San Lorenzo, silent and scarcely moving. Then, as though roused by some distant summons, he raised his white shirt above his head, tilted his face towards the heavens and gazed longingly upwards.

“He had a pale, slender, almost feminine physique,” the girl reported. “He placed one hand upon his stomach and seemed enchanted by its smoothness. His expression was one of ecstasy, and nothing could have disturbed it. He cared nothing for what anyone might think. He was not normal, in the ordinary sense of the word. When I looked again, he had vanished.”

Few thought to question why the girl had been abroad at such an ungodly hour. Instead, they indulged her by allowing her to tell her story to anyone prepared to listen. By the following evening, it had been repeated hundreds of times, and the girl was no longer the only person to have witnessed the Comte de Castello’s curious behaviour.

Although, of course, none of them had.

The drama might have subsided at that point.

But a few nights later, a restless boy named Almorò found himself unable to sleep. Remembering the tale of the handsome young count, he scrambled from his bed and perched cross-legged upon the ledge of his bedroom window, which overlooked the Ponte Cavagnis.

He hoped to catch a glimpse of the count who had become the subject of so much gossip.

It must have been three o’clock in the morning, for he heard the distant chimes of a church bell. This was significant, because he had never heard those particular chimes before, nor did he hear them again.

The final note had scarcely faded when Almorò detected movement at the foot of the steps. Anxious not to attract attention, he held his breath and fixed his gaze upon the bridge.

Out of the shadows emerged a boy considerably older than himself.

He climbed the steps slowly, placing one foot before the other, until he reached the summit. There he paused, resting both hands upon the ironwork.

In the moonlight, Almorò could discern the boy’s short, fair hair, fine-featured face and agreeable appearance. But it was his shirt that caught the young observer’s attention most. Immaculately white and fastened with small buttons, it was carefully undone, revealing a chest whose pallor suggested alabaster.

The boy caressed it with an almost devotional tenderness.

As though aware that he was being watched, he turned slowly towards Almorò and smiled. From his pocket he drew a small stone, held it flat upon his palm, and cast it as far as he could into the canal.

All Almorò saw was the spreading ripple, followed by a succession of smaller disturbances, each one setting another in motion.

When he looked back towards the boy, the bridge was empty.

Overcome by the intensity of his experience, Almorò told his parents that the boy had been beautiful. His father was quick to remind him that such a description was entirely inappropriate. Boys, he explained, could be handsome, but never beautiful.

The sensible course would have been to remain silent. Unfortunately, Almorò’s mother made the mistake of sharing her son’s extraordinary experience on social media.

It must have been the Comte de Castello, she wrote.

By the end of the day, the post had been shared thousands of times, and a remarkable number of people claimed to have witnessed the same inexplicable phenomenon. As before, however, none of them had. But it made a good story.

Over the following weeks, the narrative grew until the Carabinieri and Polizia Locale calculated that there was scarcely a soul left in Venice who had not seen the boy. Such was the public concern that additional officers were assigned to discourage the inquisitive from wandering the streets in the hope of catching a glimpse of him.

It was during one of these nightly patrols that two young officers approached the hushed Rio di Santa Maria Formosa, near the spot where John Singer Sargent had once painted one of his celebrated Venetian watercolours.

As they neared the Ponte del Mondo Novo, one officer seized the other by the arm, preventing him from taking another step. He pointed towards the bridge, where a figure stood utterly motionless.

It was a teenage boy, apparently absorbed by the murky water below, as though studying his own reflection.

The two officers stood transfixed. Neither dared to move, and neither called out.

The fair-haired boy might have been a ghost. Equally, he might have been an apparition of quite another kind: a vision of such extraordinary beauty that the distinction hardly seemed to matter.

He moved delicately, undoing the buttons of his shirt to reveal a lean, bare chest. His pectoral muscles sloped from the collarbones, curved towards the sternum and tightened with each breath. Then, with an unhurried deliberation, he caressed both nipples.

“It is the Comte de Castello,” one officer whispered.

“Shush,” replied the other.

But the boy had heard them.

Disturbed from his reverie, he turned and extended the palm of his hand. Resting upon it was a small stone that glowed an inexplicable shade of lilac.

He studied it for a moment and then, for the first and last time, he spoke.

“I know that I am beautiful, and that is good enough for me. Who are these people to doubt my intentions? If they want me, they cannot have me.”

With that, he dropped the stone into the Rio del Mondo Novo.

The two officers agreed upon one significant point. Having cast the amethyst into the water, the boy had disappeared. Neither could explain where he had gone, nor, more importantly, how. One moment he had been there; the next, he was not.

They agreed that the incident should never be mentioned.

They were unaware, however, that a dozen pairs of eyes had watched from darkened windows and witnessed the same phantasm. Those who had seen it longed to speak, but lacked the courage to do so.

There were further sightings of the Comte de Castello, each less convincing than the last. In time, as so often happens, the furore subsided, until nothing more was recorded. The elegant boy faded into folklore, becoming a story that descendants might one day discover in a book devoted to supernatural occurrences.

There was, however, an interesting twist to the tale, of which the Venetians remained entirely unaware.

On a bleak November morning, a Carabiniere with whom you are already acquainted arrested a teenage boy near the Ponte di Rialto. His offence was the time-honoured one of relieving careless tourists of their possessions as they shuffled, unsuspecting, over the steps beside the Canal Grande. A search revealed a collection of stolen wallets, purses and mobile phones which he had been unable to pass on to an accomplice.

Such misdemeanours were commonplace and tedious for law-enforcement officers. Catching the offenders, however, was not always so routine.

The Carabiniere called for a police launch to take them to the Comando Stazione Carabinieri Venezia Cannaregio. After handcuffing his prisoner, he escorted him to the waiting vessel, where they made the short journey to the station.

The boy in custody was seventeen-year-old Mani di Velluto, a native of Trento who had come to live with cousins in Mestre, a family already notorious for its misdeeds.

A personable boy, and somehow familiar, the officer mused.

He remarked upon this to his prisoner, adding that there was still time for him to mend his ways and lead an honest life.

Velluto gazed back at him with wide, innocent eyes.

“I am surprised that you caught me,” he said. “I am usually rather good at disappearing.”

The Carabiniere paid little attention to the remark. Then the boy said something that caused him to look up sharply.

“If they want me, they cannot have me.”

The officer had heard those words before.

He turned towards Velluto.

The boy had disappeared.