On the day before the wedding, Don Pedro announces to Leonato, Benedick, and Claudio that he will be leaving after the marriage of Hero and Claudio, taking Benedick with him. Benedick admits to the others that he is a changed man, and the other men — all in on the plot to match him up with Beatrice — tease him about his sad expression and his newly shaven face, clear signs that he is in love. He asks to speak to Leonato privately, and they leave. Claudio comments to Don Pedro that Beatrice has by now overheard the contrived conversation between Hero and Margaret.
Don John enters and asks to speak to his brother, suggesting Claudio will want to hear what he is reporting. He tells them that Hero is "disloyal" to Claudio, letting herself be disgraced by another man. Claudio cannot believe this report, and Don John asks that they meet him at night so he can show them what is taking place in Hero's room, even this night before the wedding. Claudio vows to shame Hero publicly if in fact she is guilty of such misdeeds.
In this scene, Don Pedro's plot to bring Beatrice and Benedick together overlaps with Don John's plot to disrupt the marriage of Claudio and Hero. Benedick has begun to recognize his love for Beatrice and has taken some tangible actions — like shaving off his beard that Beatrice does not like — to further the relationship. He may be taking Leonato, as Beatrice's nearest of kin, aside to ask his permission to marry Beatrice. Don John takes the next step in his deception first by accusing Hero of infidelity and then by offering to prove it to Don Pedro and Claudio by letting them "note" her treachery that very night. He uses a cruel sarcasm in his accusation of Hero: "Leonato's Hero, your Hero, every man's Hero."
This moment is the second time Claudio has done an about-face on Hero's loyalty based on Don John's information: First, at the dance, he believed that Hero agreed to marry Don Pedro because Don John said so; now he is ready to believe Don John again and, of course, will be convinced when he sees the window scene Don John and Borachio have arranged.
Although Don Pedro was invited to remain at Messina for at least a month, he announces that he will leave for Aragon after the wedding of Claudio and Hero. He may feel that he and his men will be in the way of the newlyweds, or perhaps he never intended to stay too long. He suggests that Benedick accompany him since Benedick has no apparent reason to stay — probably a test of whether his plot to bring Benedick and Beatrice together is having any effect. Benedick immediately shows signs of his change of heart toward Beatrice, but he becomes impatient with the teasing by his friends and leaves to talk with Leonato.
Before Claudio even sees what he thinks is a faithless Hero, he threatens a public humiliation of her at the impending wedding. The audience might wonder why he would do so publicly, reflecting poorly on himself in the process — he is being cuckolded — as well as on Hero. Why not just walk out or write her a note? But no, a public confrontation is what the immature Claudio chooses — resulting in a much better story than if he just walked away.