João, duke of Queluz, is the prime minister of Portugal. Although he is a cardinal, he is a freethinker, liberal in political and sexual matters alike. His endeavors are dedicated to the cause of progress, but in a world in which tyranny is the norm and the church attempts to exterminate all dissent, his efforts must be clandestine. His principal rival in the struggle for political power is the chief of police, the brutal Marquis da Silva. Da Silva also lusts after Alexandra, João’s niece.
In the Alfama—the district of Lisbon that plays host to social dissidents of all kinds—scientists mingle with magicians, healers with heretics, and thieves with revolutionaries. A scholarly sorceress named Clara offers shelter to a newly arrived family of refugees headed by Count Kotor. Kotor is searching for the secret of immortality, having already attained a problematic kind of longevity by virtue of being a vampire. Since his resurrection to undeath 285 years ago, he has been hunted by the Inquisition, which regards him as an instrument of Satan.
The poor people of the Alfama are ready and able to recognize Kotor as an ally. Many bring their sick to him so that he can help them cheat death. They gladly offer their blood in return. Rumors of his activities attract the attention of the secret police. Clara seeks João’s protection on Kotor’s behalf, and João is glad to lend what help he can, although it endangers his own position. He becomes more deeply committed when he falls in love with Barbara, Kotor’s daughter. Alexandra becomes similarly enamored of Laurent, Kotor’s son.
Having obtained vital information by torture and bribery, the Marquis da Silva seizes the opportunity to obliterate all of his enemies at once and unleashes a bloody invasion of the Alfama. His troops burn the coffins in which the vampires must rest by day, and they slaughter the vampires’ human allies. João manages to escape with his own life and to save Barbara, and Clara and Kotor take a ship for the New World. Da Silva manages to trap Laurent and Alexandra, but in destroying them he seals his own fate.