At his palace in Vienna, Duke Vincentio makes a big speech about how a lord in his court, Escalus, is the wisest and most knowledgeable guy in Vienna – he knows more about Vienna's laws and people than anybody else.
The Duke announces that he's going out of town. While he's away, Angelo's in charge.
While the Duke's gone, Angelo should make sure the people of Vienna don't step out of line – if they do, he has full authority to punish them according to the law (even if it means carrying out the death penalty). Escalus, an old lord, will be second in command because he's so wise and knows so much about Vienna's laws and its people.
(If you're wondering why wise old Escalus isn't first in command, you're not alone, but we'll find out why soon enough.)
Angelo goes through an "awe, shucks" routine and says he hasn't demonstrated that he's worthy enough to fill in for the Duke.
"Nonsense," says the Duke, who announces rather cryptically that he has to go somewhere immediately (he doesn't even have time for Angelo to escort him to the edge of town) and that he'll be in touch soon.
Before he leaves, the Duke reminds Angelo that while he (the Duke) is away, Angelo is his substitute and has full authority to enforce the laws or to bend the rules of justice as he sees fit.
He also adds that, even though he loves his people, he's not big on public appearances, especially since people are always cheering for him like he's a rock star.
History Snack: Some literary critics think the Duke's whole "I really don't like the limelight" speech is a reference to King James I, who, unlike his predecessor Queen Elizabeth I, wasn't a huge fan of being in the public spotlight all the time.
The Duke exits and Escalus and Angelo make plans to talk so they can work out the details of their new jobs.