At Gloucester's castle, Oswald has arrived to deliver Goneril's letter to Edmund. Regan tries to wheedle information out of him and learns that Albany's troops are on the move, seemingly at Goneril's insistence. Mostly, though, she's curious about the letter her sister has written to Edmund. She wants to know what it's about, but Oswald says he doesn't know.
Regan tells Oswald that Edmund isn't there. She thinks he's gone after his pop, Gloucester, to finish him off—and to figure out how big the enemy army is. She tries to get Oswald to stay the night instead of going after Edmund, and then she tries to get him to just show her the letter. Please? She'll be his best friend...
Oswald hesitates, and Regan says fine. She knows what's what. It's pretty obvious Goneril doesn't love her husband, and Regan noticed her flirting with Edmund before. She presses Oswald to confirm her theory, but he stalls.
Regan tells Oswald to listen up if he knows what's good for him. First off, Regan and Edmund have already talked and they agree it makes much more sense for Edmund to marry Regan—not Goneril. Second, she also has a note for Edmund that she wants Oswald to deliver. Third, when Oswald sees Goneril again, he should tell her to wise up and use her head next time instead of trying to go around her sister.
Finally, Regan informs him that there's a reward for killing Gloucester. Oswald says he's eager to kill Gloucester to prove his loyalty.