Isabella's family knows that her uncle and her are close. She says, "Look out her uncle, and you're sure of her: Those two are ne'er asunder; they've been heard in argument at midnight; moonshin nights are noon days with them; they walk out their sleeps." Even though she is young and in talks to be engaged to another young noble person, her uncle still wants her. It is said that "they're like a chain, draw but one link, all follows." It does not help that she does not want to marry her potential betrothed.

Leant is not of the upper class, though Bianca is. He says, "I have brought her from parents great in wealth, now more in rage," which shows that they were not happy about the union. His insecurity is one reason why he asks his mother to keep her in the house.

When Hippolito confesses his love for his niece Isabella, he says, "I love thee dearlier than an uncle can." She does not understand his meaning, though, and she replies, "Why so you ever said, and I believ'd it." He silently bemoans her innocence but knows that he has to be direct and upfront with her. He does not care that she is his blood relative. He wants to be with her sexually. He tells her, "As a man loves his wife, so love I thee."

This leads to Livia convincing Isabella that she is not his real niece so that she will sleep with Hippolito.

Bianca tries to resist the Duke when she meets him at Livia's house, but he argues that he is the best choice. He also points out that he could command her to obey but prefers to be gentle and kind because of his feelings for her. He says that his position and power make him someone that she should be involved with. He says, "But I give better in exchange; wealth, honour: she that is fortunate in a duke's favour, 'lights on a tree that bears all womens' wishes: if your own mother saw you' pluck fruit there, she would commend your wit, and praise the time of your nativity: take hold of glory." Eventually she gives in, though she resents Livia for setting up their meeting.

When the Cardinal speaks to his brother, the Duke of Florence, the Cardinal says that he must marry Bianca. The Duke is committing adultery and having sex outside marriage. These things threaten his relationship with God. The Duke agrees to make things right, saying "And wise men know there is no barren place threatens more famine, than a dearth in grace." Of course, his solution is to murder Bianca's husband—which is hardly more pleasing to God—but neither man stops the plot from going forward.

After a majority of the cast is dead, the Cardinal reflects on what happened and points out that lust was the downfall of everyone involved. He says, "Two kings on one throne cannot sit together, but one must needs down, for his title's wrong; so where lust reigns, that prince cannot reign long."