The full title of Bryusov's novel gives a good idea of the substance of the opera:
The Fiery Angel; or, a True Story in which is related of the Devil, not once but often appearing in the Image of a Spirit of Light to a Maiden and seducing her to Various and Many Sinful Deeds, of Ungodly Practices of Magic, Alchymy, Astrology, the Cabalistical Sciences and Necromancy, of the Trial of the Said Maiden under the Presidency of His Eminence the Archbishop of Trier, as well as of Encounters and Discourses with the Knight and thrice Doctor Agrippa of Nettesheim, and with Doctor Faustus, composed by an Eyewitness.
Set in sixteenth-century Germany, the story concerns a young woman named Renata who has fallen in love with a "fiery angel" whom she believes appeared to her during her childhood. She later identifies the "angel" with one Count Heinrich, who takes her as his mistress but then abandons her. She enlists the knight-errant Ruprecht in her efforts to track down her "angel". Though she believes her motives are holy, the presence of Faust, Mephistopheles and assorted demons in the story suggests otherwise. Her obsessive quest leads her and the loyal, lovestruck Ruprecht deep into the realms of the occult. After many bizarre adventures, Ruprecht emerges unscathed, but Renata is condemned by the Inquisition to a fiery death at the stake.