Ferdinand's talking to the Cardinal, and boy is he not a happy camper.
He's received the letter from Bosola revealing the Duchess has just had a child.
And Ferdinand's beyond furious. He'd have been angry enough if he'd just heard that she'd remarried, but since the marriage hasn't been discovered yet he thinks the Duchess has just given birth to an illegitimate child.
Ferdinand is really worked up—he's imagining his sister hooking up with all sorts of guys ("strong-thighed bargemen," for instance. Who should really have a charity calendar, like firemen), and wants to kill her.
The Cardinal stays calm, even though he's angry, too. "You've got to get it together and stay cool if you want to get revenge," he tells Ferdinand.
The Cardinal can save his breath. Nothing can keep Ferdinand from his wrath, and he exits while promising to figure out a way to "fix [the Duchess] in a general eclipse" (2.5.79). Which sounds bad.