He decides to send on Earth among the mortals his archdevil Beelphegor, disguised as a human and assisted by the invisible lower demon Adrammelech, to sabotage the peace and renew hostilities between the Pope and the Signoria.

Beelphegor starts his mission, improvising his first moves and indulging to curiosity toward experiencing carnal pleasure like eating, drinking, and sleeping with women - vices which as a spiritual entity he had been using to tempt humans over the centuries, but he could not really understand, until this first occasion to have a physical body.

Maddalena is willing to obey her father's plans for an arranged marriage motivated by politics, but she clearly affirms her independent character, promising the future husband neither love nor blind obedience.

Gianfiliazzo, the leader of Lorenzo's army and guards, is instead envious of Cibo because of his own love for Maddalena, which does not have a chance due to the disparity in social rank and her already arranged marriage with the Roman noble.

Beelphegor however escapes easily, taking advantage of both Adrammelech's invisible and supernatural assistance, and of the innovative war machines and other engineering marvels by Leonardo da Vinci.

Although most of the Florentine court seems driven to avenge the provocation with a new war, the wise Lorenzo is wary of the danger to give up the peace, weakening through further infight both the two small Italian city-states, until some stronger foreign power could invade the whole country.

Beelphegor however decides to use his remaining days as a mortal, in order to pursue his own personal agenda of seducing Maddalena as well: he had been impressed by her strength of character, and refuses to leave without adding her too to his list of conquests.

Suddenly Beelphegor steals the friar's clothes and, under pretension of giving up on the general populace and lend instead his spiritual support directly to Maddalena, easily enters Lorenzo's palace in spite of all the guards.

He escapes again easily from the furious Gianfigliazzo, this time passing through a secret passage in the bedroom of one of Lorenzo's many lovers, and again one of the genial but still untested machines of Leonardo to fly away above the crowd.

The fight turns into love-making and true affection: Maddalena no longer refuses Beelphegor, and the archdevil appears to have acquired new sentiments, at odds with his nature of evil entity.

As he starts to scream for help, Adrammelech cannot intervene but Maddalena decides to save him, even if this means joining her lover on the fire in order to force Lorenzo to halt the execution.

The angry Gianfigliazzo is pushed into the fire and finally dies; Beelphegor gratefully acknowledges the intervention of his past servant and friend, he delivers his farewell and leave the scene in loving embrace with Maddelena, fully embarking into life as a normal man with no special powers but at peace with the limitations and joys of mortality.