Measure for MeasureDuke Vincentio decides to transfer the government of Vienna to his deputy Angelo, assisted by Escalus, purporting to leave on a mission.
In fact, he stays in the city, disguising himself as Friar Lodowick. He explains to Friar Antony that he has left it to Angelo to implement a strict moral code on his behalf. Angelo enthusiastically begins the strict enforcement of the laws, including the death penalty for fornication.
This affects Claudio, who has made his intended wife Juliet pregnant, and he is arrested for lechery. Claudio asks his friend Lucio to approach Isabella, his sister who has just entered a convent, to intercede for him. At the same time, a constable Elbow brings Pompey before Escalus, on suspicion of keeping a brothel. Isabella approaches Angelo, and asks him to be merciful.
Angelo refuses, but is taken with Isabella's beauty, and tells her to return. He offers to release Claudio if Isabella will sleep with him, but she refuses. Meanwhile, Friar Lodowick is on his way to visit the prisoners when he meets Juliet and hears about her and Claudio's love.
The Friar overhears Isabella telling Claudio of Angelo's proposal. Claudio appals her by asking her to agree. The Friar intervenes, suggesting that she meet with Angelo, but that Mariana, a girl Angelo had jilted some years before, should take her place. The Friar arranges a meeting between Isabella and Mariana, who agrees to the plan.
Elbow is taking Pompey to be seen by Angelo, when they encounter Lucio. Pompey asks Lucio for bail, but Lucio refuses. The Friar is present, and engages Lucio in conversation, asking him what he thinks of Duke Vincentio. Lucio paints a black picture of the Duke's character. Escalus and the Provost take Mistress Overdone to prison. Escalus expresses his sympathy for Claudio to the Friar. The Provost gives Pompey a job assisting Abhorson, the executioner.
Both Claudio and an unrepentent murderer, Barnardine, are due to die the next day. Friar Lodowick is present when a message arrives from Angelo; but instead of it being the expected pardon, it is an instruction for Claudio to be executed early in the morning, and for the head to be brought to him. Following the Friar's suggestion, the head of a dead prisoner is substituted for Claudio's. Isabella arrives expecting to be told of Claudio's release.
Instead, the Friar tells her that Claudio is executed, and that she should obtain justice from the Duke, who is about to return. Duke Vincentio then arrives in the city, and is approached by Isabella, who accuses Angelo of his attempted seduction. The Duke pretends not to believe her, and has her arrested.
To support her, Mariana reveals herself and affirms that she has slept with Angelo. Friar Lodowick is called for, and the Duke leaves, returning as the Friar to defend the women. Condemned by Angelo as a false witness, he reveals himself.
Angelo admits his guilt, but is pardoned on condition that he marries Mariana. Claudio is reunited with Juliet, and the Duke proposes to Isabella.