The action of LA FIAMMA is set during the conflicts between the Eastern Roman and the Western Roman Empires, at a time when large parts of Italy belonged to the Byzantine sphere of influence. The Exarch Basilio has been appointed the representative of Eastern Rome, having left Constantinople many years before with his family at the Emperor’s command. In the meantime, his first wife has died and his son Donello is studying in Constantinople. Finally, Basilio married Silvana, who is much younger than he and from a simple background, a marriage his mother Eudossia opposed. Silvana has now been living in her husband Basilio’s house for several years, together with her mother-in-law, who never got over her antipathy for the young woman.

Act I

Once again, Eudossia reprimands her daughter-in-law for not running the household with the iron hand she considers necessary. She paints her deceased predecessor as a model of discipline, apart from the advantage of having been an honourable noblewoman. Silvana seems to be suffocating in the atmosphere her mother-in-law creates; she confides in Monica, one of the household’s servant girls. At that moment, angry shouting from a crowd of people is heard, demanding that Agnes di Cervia be condemned to death by fire. They claim that she is a witch. When the servant girls rush out to join the crowd, the old woman they are hunting suddenly appears before Silvana, scared to death and begging her to save her and shelter her from the mob. At first, Silvana refuses, but the moment Agnese starts to hint that Silvana’s mother was once accused of witchcraft too, Silvana decides to give in to her plea and hides her.

In the meantime, Donello, Basilio’s son from his first marriage, has returned to Italy. He is welcomed formally by his stepmother Silvana. They are the same age, and Silvana reminds him that they have met once before, when they were still almost children. Even then, they seemed to be drawn to each other, as they are now at their reencounter. Eudossia interrupts their conversation, welcoming her grandson Donello.

At this moment, the crowd pursuing Agnese di Cervia storms into the house, led by the exorcist of the nearby church. They accuse Agnese of having bewitched young Cesario and being responsible for his death. Traces have led them to the Exarch’s palace, and they think she is hiding here. Finally, the mob finds the old woman and drags her off to the pyre. Agnese curses strict Eudossia, her son Basilio and her grandson Donello, but also Silvana, to whom she prophesies that she too will end up tied to the stake one day.

Act II 

The next day

Following his arrival, Donello has started an affair with the servant girl Monica. When Silvana learns of this, she displays uncharacteristic strictness and banishes her friend Monica to a monastery. Basilio, on the other hand, orders his son to join him in a campaign against the Pope in Rome. They all seem affected by Agnese di Cervia’s curse. Shortly before she succumbed to the flames, she made strange hints about Silvana’s mother and her connections with the Exarch. Basilio finally confesses to Silvana that her mother enticed him into her house by using magic, there setting him up with her own daughter, who was not even of age at the time. He immediately succumbed to her charms, but always truly loved her as well. When Silvana’s mother was accused to witchcraft years later, Basilio defended her and saved her from death by fire. However, he is still convinced that at the time, she put a spell on him, so now he suffers from the notion that according to his faith, he saved the sinner from the pyre, but not from the purgatory of hell. Confused, Silvana now begins to understand why she never feels grief at her mother’s death. At the same time, she believes she has inherited her mother’s magic powers, so that what she longs for in her dreams might come true, only because of her power of imagination. She tries to make her stepson appear, and he suddenly stands before her. They fall into each other’s arms and spend the night together.

Act III 

Several months later

Silvana and Donello have continued their fateful affair and cannot keep away from each other. Eudossia has long discovered the affair, but has kept this knowledge to herself to spare her son Basilio. However, she has used her political influence to ensure that Donello is ordered back to Constantinople. On the day when she wishes to inform her grandson of this, she finds Silvana in her grandson’s room. Their adultery can hardly be kept a secret anymore. When Silvana understands that Donello is to be separated from her due to an intrigue of her mother-in-law’s making, she is beside herself with rage and desperation and openly confesses her adultery to Basilio. Moreover, she tells him she has not had one happy moment by his side and that she despised him every time she had to give in to his advances. Old Basilio breaks down and dies. Eudossia blames Silvana for her son’s death and accuses her of being a murderess, a witch even.

Unlike Agnese di Cervia, who was handed over to the mob’s justice, Silvana stands for a proper trial. She confesses her adultery, but rejects any accusations of witchcraft. Her defence is that she succumbed to the flame of love, and that is her only crime; that Donello and she are bound by love and have a right to live this love. When Donello also defends her, the presiding bishop and the listening crowd are almost ready to acquit Silvana. At that moment, however, her mother-in-law Eudossia speaks, reminding all those present of her connections with Agnese di Cervia, who was convicted of witchcraft, and of the rumours surrounding Silvana’s witchlike mother. Finally, she ends her plea with the words: “Witch! Daughter of a witch!” At this point, Donello also starts doubting Silvana, and she understands that she can no longer trust him. Her dream of love is fractured, and she sees no more meaning in life. When the bishop gives her the final chance to denounce witchcraft by swearing an oath, she fails to finish the words. Her silence is assumed to be a confession. Silvana is taken to the pyre, and her hope for a life in love and without superstition is destroyed.