Vincentio arrives at the city gates once again dressed as the Duke. He greets Angelo and proclaims that he has heard good things about the justice done in the city. Isabella comes forward and accuses Angelo of being a murderer and a violator of her virginity. Angelo denies it and Duke Vincentio pretends not to believe Isabella, suggesting that she might be mad. However, Isabella gives a clear and rational account of her bargain with Angelo and Duke Vincentio declares that mad people are rarely so logical. He asks for evidence, and Isabella and Friar Peter tell him that another friar called Friar Lodowick—actually the Duke’s former disguise—can confirm their testimony.

Angelo suggests that this might be a conspiracy against him and that the women are being manipulated by Friar Lodowick. Lucio agrees, claiming that Friar Lodowick is a scoundrel who insulted the Duke, when, in fact, he was the one who insulted Vincentio. The Duke proclaims that they must find the friar and interrogate him. As Isabella and Friar Peter go to fetch Friar Lodowick, Mariana steps forward and claims that she was actually the one who had sex with Angelo, who was her husband despite them not being wed.