The action is set in Sicily, where Don Pedro, Prince of Aragon, has recently defeated his half-brother, the bastard Don John, in a military engagement.

Apparently reconciled, they return to the capital, Messina, as guests of the Governor, Leonato.

There Count Claudio, a young nobleman serving in Don Pedro's army, falls in love with Hero, Leonato's daughter, whom Don Pedro woos on his behalf.

The play's central plot shows how Don John maliciously deceives Claudio into believing that Hero has taken a lover on the eve of her marriage, causing Claudio to repudiate her publicly, at the altar.