A brief summary.
Measure for Measure a comedy by Shakespeare. It was probably first acted in 1604, but not printed till the folio of 1623. The plot is taken from Cinthio (translated by Whetstone).
The duke of Vienna, on the pretext of a journey to Poland, hands over the government to Angelo, that he may escape the odium of enforcing laws against unchastity that have long been disregarded. Angelo at once sentences to death Claudio as guilty of seduction. Claudio sends words of his position to his sister Isabella, a novice, and begs her to intercede with Angelo.
Isabella's prayers fail to win her brother's pardon, but her beauty awakens Angelo's passion, and at a second interview, he offers her her brother's life if she will sacrifice to him her honour. Isabella indignantly refuses; and there follows the famous scene in the prison, when Isabella tells her brother of Angelo's offer, and he, momentarily weakening, pleads with her for his life.
Meanwhile the duke, who has not left Vienna but assumed the disguise of a friar, and thus learnt the infamous conduct of Angelo, contrives the saving of Claudio as follows: He bids Isabella consent to go to Angelo's house at midnight, and obtains that Mariana, who had been to Angelo and loves him, but had been cast off by him, shall go there in Isabella's place. The ruse is successful; but none the less Angelo orders Claudio's execution at dawn.
The duke, laying aside his friar's robes and simulating an unexpected return to Vienna, hears the complaint of Isabella and the suit of Mariana, and confutes Angelo, who denies her stories. Angelo is pardoned at the instance of Mariana and Isabella, and married to the former; and the duke reveals his love for Isabella. (Oxford Companion to English Literature)