Don John and Borachio are freshly sulky over the news of Hero’s wedding to Claudio. Borachio says he’s figured out a simple and fool-proof way to ruin the marriage, which would make Don John really happy.
Borachio reminds Don John of Hero’s attendant, Margaret, who he’s apparently been screwing around with for some time.
Borachio talks vaguely of a plan to have Margaret stand in Hero’s window. Don John, however, is slightly confused about how exactly this is a foolproof plan to ruin weddings and lives.
Borachio’s got it all planned out: all Don John has to do is go to Don Pedro and announce that he’s discovered Hero is actually in love with Borachio. He’ll need to pretend to be apologetic that Claudio’s future marriage is ruined, as is Don Pedro’s reputation as a matchmaker and an honorable man. Don John can insist he’s only telling Don Pedro out of love.
Surely, Don Pedro will require proof of this slander, and that’s where big deception comes in.
The night before the wedding, Don John should bring Don Pedro and Claudio to Hero’s window. Borachio will have contrived to make Hero absent, and Margaret will stand in silhouette by the Hero’s window, appearing to be Hero. There, Borachio himself will be making love to Margaret (either in word or deed) while calling her "Hero." The men will witness this, and it will seem like solid visual proof that Hero is cheating on Claudio. That should be enough to ruin the wedding.
Don John promises Borachio a thousand ducats in payment for this scheming, and the two men part to set their plan in action.