Duke Vincentio has given his deputy Angelo temporary power over Vienna before his leave for a diplomatic mission. But instead of leaving the city, the Duke disguises himself as a friar named Lodowick, and stays behind to observe the city's happenings and Angelo's brief reign.
Meanwhile, Angelo decides to enforce the laws on prostitution and debauchery, effectively shutting down the red-light district. A man named Claudio gets arrested for impregnating his lover, and Angelo sentences him to death. Claudio's sister enlists a certain friar's help to trick Angelo into releasing her brother.