The Duke and Isabella pay Mariana (Angelo's ex-fiancé) a little visit at her farm, which is surrounded by a moat. When they arrive, Mariana is listening to a Boy singing a sad song about a jilted lover.
When Mariana sees the Duke (still disguised as a friar) she tells the kid to scram so she can talk in privacy (apparently, the Duke has visited with Mariana before and listened to her confession).
Isabella and the Duke confer about where and when her secret rendezvous with Angelo is supposed to go down – Isabella is supposed to meet Angelo in a secret garden.
Isabella and Marianna take a walk together during which time Isabella fills in Marianna on the Duke's plot to trick Angelo.
Marianna is all for the Duke's little bed trick.
Brain Snack: "Bed trick," by the way, is a common term for one of Shakespeare's favorite plot devices – it always involves one person who thinks he/she is going to bed with another person, but who is then tricked into sleeping with someone else. In Shakespeare, the duped party is usually a guy, like Bertram in All's Well That Ends Well.
The Duke explains that Marianna's actions won't be counted as a sin or a crime because Angelo is her "husband on a pre-contract," meaning, they were formally betrothed before Angelo backed out of the wedding at the last minute.
Everybody runs off to put the sneaky plan into action.