Based on a novel by Ben Hecht, a man obsessed by the Borgias falls in love and is assisted by a psychiatrist in finding the murderer of her father.
Dr. Lytton (C. Aubrey Smith), Victor Ballau (Henry O'Neill), and Cesare (Donald Woods) visit Rossano and the Borgia castle. Cesare orders a prescription and drinks to the Borgias to die. Psychiatrist Lytton says that Cesare is paranoid and needs help. Cesare says he is the last Borgia and is afraid he will kill if he doesn't die; but Dr. Lytton says it was not poison. He suggests Cesare work it out by writing a play.
In Vienna Cesare rehearses his play produced by Victor. He finds Florence (Margaret Lindsay) arguing with Victor and says that she is Lucrezia. Cesare tells Dr. Lytton that his play is a hit and that he is in love; but Dr. Lytton warns him to be careful. Cesare tells Florence he is going to her father to ask for her hand; but Victor refuses. Cesare picks up the Borgia's dagger. Cesare learns that Florence left during the play. Cesare goes to Florence and finds Victor dead by the dagger. Police captain (Robert Barrat) questions housekeeper Teresa (Florence Fair), Florence, and Cesare. Florence won't see Cesare, and Dr. Lytton tells Cesare that she is suspected. Cesare sees Florence leaving and follows her to the theater. Cesare asks if he or she killed Victor. The captain arrests her and says that Victor was not her father but step-father. The captain says that Victor promised he would end his romantic attentions she loathed but that he threatened her with exposure. Cesare tells Dr. Lytton that she is shielding him but is persuaded not to turn himself in.
In jail Dr. Lytton hypnotizes Florence, who says she did not kill her father but was there. Dr. Lytton tells the captain that Florence was acting. Dr. Lytton is reading late when he is stabbed by a hooded person. He tells Cesare that it was a woman. Dr. Lytton finds the captain and his men searching his house. The captain says that Florence was released, but they lost her. At the auction of Victor's property Fredericka (Eily Malyon) buys the candlesticks and is followed by Cesare and Dr. Lytton, who are followed by police. Fredericka tells Cesare and Dr. Lytton that Victor deserved to die. Cesare finds Florence, who cries. Dr. Lytton calls Teresa for Fredericka. Teresa arrives and says that she is Florence's mother and killed Victor for harming her in a fire and for trying to stop Florence from marrying Cesare. The captain listens and orders the recording destroyed. In the final scene Cesare, Florence, and her mother fly off in a plane.
This mystery recalls the Borgia's crimes, but it is a modern psychiatrist and a clever police captain who uncover the sordid past that ended in murder. Such incest is often an enduring and undetected crime.