The Duke greets Angelo and Escalus at the city gates. He thanks them. Friar Peter enters with Isabella and tells her to speak to the Duke. She begs him for justice. The Duke tells her to state her complaint briefly to Angelo. Isabella says that she cannot ask Angelo for help, because he is evil. She wants to speak to the Duke directly. Angelo interrupts, trying to tell the story himself, but Isabella continues, calling Angelo a murderer, hypocrite, "adulterous thief," and "virgin-violator." The Duke tries to send her away, calling her insane.
Isabella asks the Duke to reconsider, arguing that even someone who seems noble, like Angelo, can actually be bad. The Duke realizes that Isabella is far too logical to be insane. Isabella urges him to see reason. The Duke asks her to tell her story, and she begins by recounting how her brother was sentenced to death for fornication, and how she was asked by Lucio to ask Angelo for his pardon. Lucio verifies this, but the Duke tells him not to speak. Isabella goes on to say that Angelo asked her out of lust to have sexual intercourse with him in exchange for her brother's life. She says that she obeyed, but that Angelo sent the warrant for Claudio's execution anyway. The Duke does not believe her, saying it is illogical that Angelo should have acted in such a way. He asks Isabella to confess that she is lying, and to name the person who sent her.
Isabella prays to heaven to make the truth known. The Duke orders her sent to prison. He asks her who knew of her coming, and she names Friar Lodowick. The Duke asks if anyone knows this friar, and Lucio says that he knows him but does not like him. He also accuses the friar of slander against the Duke and claims to have silenced him. The Duke demands to see the friar. Friar Peter comes forth and says that he has heard everything, agreeing that Isabella is lying. The Duke asks him if he knows Friar Lodowick, and Peter says that he does know him, and that Lodowick is a good man who has never said anything bad against the Duke. He says that Lodowick is sick and has sent him in his place. He is meant to share Lodowick's knowledge, beginning with the fact that Isabella is lying. Isabella is led away by guards, and Mariana enters, veiled, as a witness.
The Duke asks Mariana to show her face before testifying, but she says she will not lift her veil until her husband instructs her to. He asks if she is married, and she says no. He asks if she is a maid or a widow, and she again says no. Confused, the Duke asks Mariana to explain. She tells him that she has had intercourse with her husband, though he does not know it, and therefore is not a maid. The Duke says that her testimony is irrelevant to Angelo's case, but she names Angelo as the husband she is speaking of. Angelo denies it and asks to see her face.
Mariana lifts her veil. The Duke asks if Angelo knows her, and Angelo confesses that he was engaged to her five years before. He swears that he has not seen her since. Mariana explains that they had sexual intercourse on Tuesday night. Angelo objects again, saying that both women are crazy and being exploited by some other person. The Duke sends for the other friar. Friar Peter tells him that the provost knows where he is, so the Duke sends the Friar to find him. He then leaves, telling Escalus to continue listening to testimony.
Escalus calls for Isabella, saying that he wants to question her himself. Lucio advises him to question her in private, suggesting that she might be ashamed to speak the truth in public. Escalus tells Isabella that someone has denied what she said, attempting to get the truth out of her.