Everyone is gathered at the city gates as the duke arrives. Isabella accuses Angelo, calling him a 'virgin-violator'. The duke pretends not to believe her and has her arrested. Mariana is brought in and reveals the bed-trick. Angelo admits he knows Mariana but says both women are mad and must be 'instruments of some mightier member' who plots against him. Isabella mentions ‘Friar Lodowick’ and Lucio immediately accuses the 'meddling friar' of slandering the duke. The duke sends for Friar Lodowick and then leaves for a while while Escalus questions Lucio further. When the duke re-enters, disguised as Friar Lodowick, Lucio tears off his hood, revealing the duke himself. Angelo confesses his 'shame' and begs for death but the duke sends him away to marry Mariana.
The duke apologises to Isabella for not preventing Claudio’s death. Angelo and Mariana are brought back in, married, and the duke orders Angelo’s execution to make things even: 'measure still for measure'. Mariana asks Isabella to plead for her husband’s life and Isabella agrees, saying that 'sincerity govern’d' Angelo until he met her. The duke then reunites Isabella with Claudio, who is still alive, and asks her to marry him. The duke pardons Angelo and orders Lucio to marry the woman he made pregnant. The duke offers himself as a husband to Isabella again. Isabella does not reply.