Mariana is listening to a boy singing a song when Vincentio arrives in his disguise. Isabella enters and the three of them conspire about how they will trick Angelo into having sex with Mariana. Isabella reveals the secret route into Angelo‘s bedchamber that he showed her and they plan to have Mariana go there instead that night, using the darkness to disguise her identity. Both women agree to the plan and Isabella instructs Mariana to remind Angelo about his agreement to free her brother. Vincentio assures them that since Angelo agreed to marry Mariana, he is lawfully her husband despite the lack of a wedding ceremony, and therefore this is not a sin.

At the prison, Pompey agrees to serve as assistant to the executioner, Abhorson, in exchange for avoiding his whipping. Pompey and Abhorson discuss the relative honesty of their professions—the bartender at a brothel and an execution—and Pompey decides that executioners are more penitent.

Vincentio returns to the prison and learns that Claudio is scheduled to be executed the next day. A note arrives from Angelo, but it is not a pardon. Rather, it confirms that Claudio should be executed alongside another prisoner called Barnardine in the afternoon and then his head should be sent to Angelo.