At Messina, a messenger brings news that Don Pedro, a Spanish prince from Aragon, and his officers, Claudio and Benedick, have returned from a successful battle. Leonato, the governor of Messina, welcomes the messenger and announces that Don Pedro and his men will stay for a month. Beatrice, Leonato's niece, asks the messenger about Benedick, and makes sarcastic remarks about his ineptitude as a soldier. Leonato explains that "There is a kind of merry war betwixt Signior Benedick and her."
Beatrice and Benedick, longtime adversaries, carry on their arguments. Claudio's feelings for Hero, Leonato's only daughter, are rekindled upon seeing her, and Claudio soon announces to Benedick his intention to court her. Benedick tries to dissuade his friend but is unsuccessful in the face of Don Pedro's encouragement. While Benedick teases Claudio, Benedick swears that he will never get married. Don Pedro laughs at him and tells him that when he has found the right person he shall get married.
A masquerade ball is planned in celebration, giving a disguised Don Pedro the opportunity to woo Hero on Claudio's behalf. Don John, "The Bastard" (Don Pedro's illegitimate brother), is a malcontent who uses this situation to get revenge on his brother Don Pedro by telling young Claudio that Don Pedro is wooing Hero for himself. Claudio becomes furious at Don Pedro and confronts him. The misunderstanding is quickly resolved and Claudio wins Hero's hand in marriage.
Don Pedro and his men, bored at the prospect of waiting a week for the wedding, harbour a plan to matchmake between Beatrice and Benedick. The men led by Don Pedro proclaim Beatrice's love for Benedick while knowing he is eavesdropping on their conversation. The women led by Hero do the same to Beatrice. Struck by the fact that they are apparently thought to be too proud to love each other, Beatrice and Benedick, neither willing to bear the reputation of pride, each decides to requite the love of the other.
Meanwhile Don John plots to ruin Claudio and Hero's wedding by casting aspersions upon Hero's character. His follower Borachio courts Margaret, Hero's chambermaid calling her "Hero", at Hero's open bedroom window while Don John leads Don Pedro and Claudio to spy below. The latter mistaking Margaret for Hero are convinced of Hero's infidelity.