Alsemero falls in love with Beatrice, but she has already been promised in marriage to Alonzo. Beatrice is also obsessed over by de Flores, the servant of her father Vermandero. Beatrice gets her father to invite Alsemero to his castle.
Beatrice decides she wants to marry Alsemero. De Flores enters and tells her that Alonzo has arrived with his brother, Tomazo. Beatrice vows to get her father to dismiss De Flores. However, after speaking to Alsemero, Beatrice plans to manipulate De Flores and get him to murder Alonzo so she can marry Alsemero. However, De Flores sees this as an opportunity to possess Beatrice sexually.
De Flores kills Alonzo, and then insists on receiving his "payment" from Beatrice, who takes some time to understand what he is saying. He insists that they are united by this crime, and Beatrice begins to understand what she has done.
Beatrice has given into the sexual advances of De Flores and is also going to marry Alsemero. She arranges for her servant to sleep with him on the first night as she is not a virgin now. The final scene is the uncovering of all of this deception, as De Flores is fatally wounded and admits his crime. Beatrice kills herself after admitting her role in what has happened.