The narrative opens on a stormy night in the Black Forest in fifteen sixteen, where an ancient and destitute shepherd named Fernand Wagner sits entirely alone after the mysterious disappearance of his beloved granddaughter Agnes. Desperate, starving, and abandoned by fate, Wagner is suddenly accosted by a strange and majestic traveler possessed of supernatural powers. The stranger offers him eternal youth, vast riches, and intellectual vigor in exchange for two specific conditions: that Wagner accompanies him for eighteen months until July seventeenth, seventeen, and that he preys upon the human race. Accepting this dark pact, Wagner drains a magical vial and is instantly transformed into a vigorous young man.
The story then shifts forward to November fifteen twenty in Florence, focusing on the dying Count of Riverola and his children, Francisco and Nisida. Nisida is a striking, passionate young woman who was left deaf and dumb following her mother's mysterious death years prior. The stern count summons his son to his deathbed, commanding him to open a forbidden closet on his wedding day that holds the key to a family secret. Following the count's demise, the funeral is dramatically interrupted by the sudden appearance of a beautiful stranger named Agnes, who hysterically mourns the nobleman, revealing that she was his secret lover. She is abruptly whisked away by a handsome stranger who is none other than Fernand Wagner in his renewed youth, now residing in Florence and operating under mysterious circumstances.
The reading of the count's will reveals that the vast Riverola estates are held in trust for Francisco, but will pass to Nisida if she ever miraculously recovers her speech and hearing. Meanwhile, Agnes dwells in Wagner's Florentine mansion, tormented by horrifying visions of a majestic and vengeful dark-haired woman who has twice terrorized her and stolen family jewels. This mysterious dark-haired woman is soon revealed to be none other than the deaf and dumb Nisida herself, who harbors a deep, vindictive hatred toward Agnes for her liaison with the late count. As Wagner and Nisida grow mutually enamored, unaware of the bloody feud between them, the fateful last day of the month approaches.
On the evening of January thirty-first, fifteen twenty-one, the terrible curse of the werewolf overtakes Wagner. Stripped of his humanity at sunset, he transforms into a ravenous wolf and embarks on a frantic, murderous rampage across the Tuscan countryside, terrifying villagers and leaving a trail of carnage in his wake. During this chaotic night, the relentless Nisida, operating under the guise of an armed cavalier, tracks Agnes to the gardens of Wagner's estate and ruthlessly murders her out of jealous vengeance. Wagner returns at dawn, horrified to discover the lifeless body of Agnes, and is immediately arrested by the Florentine authorities who find his garments blood-stained from his bestial transformation.
While Wagner languishes in a subterranean dungeon visited only by Nisida in disguise, the plot thickens with the disappearance of Flora Francatelli, Francisco's beloved. Flora is abducted by nuns and dragged down into the subterranean cells of the notorious Carmelite Convent. The Countess of Arestino is similarly imprisoned by her jealous husband after he discovers she pawned her bridal diamonds to pay her lover's gambling debts. As the narrative races toward its climax, the bandit chief Stephano Verrina and the Marquis of Orsini prepare to storm the convent to liberate the captive countess, setting the stage for a final reckoning of crime, vengeance, and supernatural retribution.
