Federico, a young man-at-arms, wants to clear the name of his brother, a priest seduced by a nun, Sister Benedetta. With this aim in mind, he goes to the Bobbio prison convent, where Sister Benedetta is incarcerated, after being accused of witchcraft. She bewitches Federico in turn. She is then sentenced to life and walled up alive.
In the same place, many centuries later...
Federico, a state inspector, knocks on the door of the convent, which had been transformed into a prison before falling into disuse. He discovers that the building is inhabited by a mysterious count, who only goes out at night.