The first scene, set at Messina, opens with Leonardo, the governor, his daughter Hero and her cousin Beatrice. Talking to a messenger, Leonardo learns that Don Pedro, prince of Aragon, is to come to Messina. He had recently fought a battle nearby, wherein a young gentleman named Claudio had distinguished himself as a brave soldier and thereby earned Don Pedro's praise. Beatrice asks the messenger for Signior Benedick and, after hearing he has returned safely, sneers at his boasting before the battle. The messenger, puzzled at her showing contempt, puts her right as to Benedick's heroic qualities, and Leonardo explains that Beatrice and Benedick have a "merry kind of war" going on between them, consisting of insulting each other in a witty way. Thereupon Beatrice denies Benedick not only any kind of esprit and pities his companion Claudio for having to be near him.
A trumpet announces Don Pedro and his followers. After having greeted Hero and her father, Don Pedro takes Leonardo aside to talk to him. Beatrice starts to attack Benedick for being impolite, who in turn calls her "Lady Disdain". He furthermore affirms that he is loved by all ladies except Beatrice, but he himself loves none, whereupon Beatrice claims not to care about love either. Don Pedro and Leonardo turn back to the company, declaring that Don Pedro, Claudio and Benedick will stay in Messina on Leonardo's invitation for at least a month. Leonardo greets Don John, Don Pedro's recently reconciled brother. Everyone exits except Claudio and Benedick.
The former asks his friend for his opinion on Hero, naming her the "sweetest lady" he ever saw. But Benedick would not praise her, but holds that her cousin Beatrice, although being constantly angry, excells Hero in beauty. When Claudio states that he would not be averse to marrying Hero, Benedick deplores men's readiness to give up their bachelorhood and warns Claudio not to willingly place his "neck into a yoke".
Don Pedro joins the two, and Benedick tells him of Claudio's having fallen in love for Hero. The prince congratulates Claudio on his choice and smiles at the disdainful attitude to women and love Benedick has, whom Don Pedro expects to fall in love soon, too. Benedick disagrees, and is sent away by Don Pedro on an errant. Alone with Claudio, the prince wants to know the nature of Claudio's feelings for Hero, whereupon Claudio confirms that he has indeed turned from a soldier into a lover. Don Pedro offers his help in revealing Claudio's love to Hero and her father by disguising himself as Claudio at the upcoming masked ball in order to win Hero's heart for Claudio by a passionate speech of love. After Leonardo has been told, Hero shall be Claudio's, Don Pedro concludes.
Leonardo meets his brother Antonio in the house and is being told that a servant of Antonio had overheard Don Pedro and Claudio in the garden. Since the man only heard the part of the dialogue where Don Pedro announced to open up his heart to Hero, the two brothers assume that it is Don Pedro himself who is in love with Hero. Leonardo declares his intention to prepare his daughter for this.
In Leonardo's guest house, Don John is asked by his follower Conrade why he is so sad without any apparent reason. When Don John claims that he cannot help his nature, Conrade reminds him of a disagreement with his brother, Don Pedro, only recently resolved, and advises Don John not to show himself an unpleasant man. However, Don John rejects the idea of flattering his brother, calling himself a "plain-dealing villian" instead. Borachio, Don John's second follower, enters and brings news of the intended marriage between Claudio and Hero, as well as of Don Pedro's plan of disguising as Claudio at the masked ball. Don John rejoices at the thought of crossing their plan in order to ruin his rival Claudio, whom he envies for his success. Conrade and Borachio plead allegiance.