Don Juan is about to leave Madrid for Granada when his friend Don Tello, with whom he sailed from Mexico to Spain, tells him he is expecting two nephews from Burgos to marry his daughters: Don Mendo will wed Leonor and Don Diego will wed Inés. Don Juan loves Inés and she loves him, but they have kept it secret.
Mosquito, the comic servant, has gone to see the suitors and comes back with a portrait of Don Diego: he got up at five and, between garters, comb, mirror and clothes, it was two in the afternoon, too late to find a Mass. On stage, Don Diego scolds his servants for not setting the mirrors “in the fashion” and swears no woman can look at him without falling in love. Don Mendo, by contrast, is sensible, and Leonor is happy with him.
Inés will not disobey her father, so in a long speech she asks Don Diego to be the one who refuses the marriage; all he understands is that both cousins are jealous over him. Mosquito then hatches a plot: to make him believe that a widowed countess, Don Juan’s cousin, has fallen for him. The real countess is away in Guadalupe, and the part is played by Beatriz, a maid dismissed from the house, who receives Don Diego in the most affected, high-flown language she can find.
The trick works only too well. The lies Don Diego improvises to escape each tight spot stir up jealousy, misunderstandings and sword challenges between Don Juan and Don Mendo, while Don Tello has the wedding ready for that very night.
The ending (spoilers)
Inés, who out of jealousy over the veiled woman Don Diego claimed was Don Juan’s lady had resolved to marry her cousin, learns it was all his invention, and Don Mendo, to whom she confessed her love to stop a duel, takes her side. Don Diego, certain he will marry the countess, tells his uncle he cannot marry Inés because he is already married, and Don Tello draws his sword. Don Mendo explains that Don Juan wished to ask for Inés’s hand, and Don Tello, who esteems him and wants to punish the fool, marries Inés to Don Juan and Leonor to Don Mendo. Mosquito then asks for Beatriz’s hand; Don Diego protests that she is his wife, a countess in disguise, and she sets him straight: “Yo descubriros no puedo / más de que soy Beatricilla, / y vos el lindo don Diego” (“All I can reveal is that I am little Beatriz, and you are dapper Don Diego”).