A messenger delivers a letter to Leonato, governor of Messina, announcing that Don Pedro, Prince of Aragon, will arrive shortly. Don Pedro and his followers have emerged victorious and almost unscathed after halting a rebellion by his own brother, Don John. The messenger reports that Claudio, a young lord of Florence, has carried himself especially well and is much honored by Don Pedro. Leonato's niece Beatrice asks the messenger about the fate of another of Don Pedro's men: Benedick of Padua. Apparently, she does not think well of him, as indicated by her scathing remarks about him. The messenger reports that Benedick also acquitted himself well in the battle and is now a companion of Claudio's.
Don Pedro soon arrives with Claudio and Benedick, as well as Don John and other soldiers. Leonato graciously receives the visitors and invites them to stay for at least a month.
Left to themselves, Beatrice and Benedick exchange taunts and insults, symptomatic of their "merry war."
Later, alone with Benedick, Claudio confides that he wants to marry Hero, Leonato's daughter. He ignores Benedick's heckling about marriage and his disparagement of Hero. Don Pedro reenters and is told about Claudio's intent to seek Hero for his wife. Don Pedro encourages Claudio and promises to approach Hero and her father on Claudio's behalf during the evening's masked celebrations.
The battle between the forces of Don Pedro and his rebellious brother Don John is alluded to quite briefly in references to the few casualties suffered and the valor of both Claudio and Benedick. The audience gets a first clue about the discord between Don John and Don Pedro when the stage direction refers to "Don John the Bastard," and a second clue when Leonato refers to John as being "reconciled to the Prince your brother." Not until Act I, Scene 3, does the audience learn how his defeat and his submissive position in his brother's circle humiliates Don John and electrifies his desire to create trouble in the household, especially for Claudio, who was instrumental in his defeat.
In this first scene, all four "romantic" young people who will remain at the center of the play's action are introduced:
Hero represents a young woman in tune with the traditions of the time — seen but seldom heard, deferential to her father, awaiting an appropriate suitor to take formal steps to court her. In keeping with her modest demeanor, Hero has only one brief line during this scene, but she is the subject of conversation as soon as she leaves the stage.
By contrast, Beatrice tends to take charge of every conversation, not reluctant to state her own views on a subject regardless of whom she addresses. Her wit and sarcasm are wasted on the messenger, who doesn't know what to make of her. Her uncle, Leonato, acknowledges her ongoing "merry war" with Benedick. Finally, she engages Benedick himself, who can give back as good as he gets.
Before he appears, Claudio is reported to be much honored by his commanding officer, Don Pedro. Like Hero, he is quiet when Don Pedro and his men are welcomed by Leonato. But when everyone leaves, Claudio immediately begins talking about his love for Hero in a very traditional manner, prompting Benedick to rant against women in general and Hero in particular. Claudio readily accepts Don Pedro's offer to speak both to Hero and to her father for him. During this planning, Claudio determines that Hero is Leonato's only heir.
Benedick too is mentioned before he appears, but only by Beatrice, who is clearly bitter toward him, apparently as the result of previous experience with him. In his battle of words with Beatrice, Benedick puts up a noble fight, finally putting her on the defensive, but while Benedick has the last word this time, Beatrice ends the conversation with an aside, spoken for the benefit of the audience, revealing that she and Benedick have known each other personally in the past and that this war of words is not something new. Regarding his attitudes about women, Benedick admits that he is a "professed tyrant to their sex."
This one scene literally sets the stage for all the main events to follow: the courtship and anticipated marriage of Hero and Claudio, the thorny relationship between Beatrice and Benedick, and the disruptive mischief of Don John.
The first instance of wordplay on "noting/nothing" appears in this scene:
Claudio: Benedick, didst thou note the daughter of Signor
Benedick: I noted her not, but I looked on her.
This dialogue introduces two meanings of the word "note": observe and study.
The groundwork for the play's first deception is soon laid: Not only will Don Pedro speak to Hero on Claudio's behalf, but he will be wearing a mask and pretending to be Claudio.
The next two scenes will reveal that the conversation between Claudio and Don Pedro has been overheard ("noted") by at least two others: by Antonio's "man" and by Borachio, one of Don John's followers. The conversation is misunderstood by Antonio's man, leading to some confusion, but correctly reported by Borachio to Don John, leading to a first deception to be set up by Don John.