The scene shifts to Southampton. We learn from Gloucester, Exeter, and Westmoreland that Henry knows all about the traitors' plot to assassinate him.
Brain Snack: Shakespeare doesn't exactly tell us how Henry found out about the plot, but the fact that he does tell us that Henry has a good spy network, which is Shakespeare's way of giving his monarch, Queen Elizabeth I, a big shout-out. (Elizabeth I was famous for having a bunch of spies everywhere. In fact, one of her mottos was Video et taceo, which is Latin for "I see and am silent.")
Meanwhile, King Henry is boarding a ship that will take him and his troops to France. With him are Scrope, Cambridge, and Grey (the three traitors), who are busy trying to brown-nose the King.
Henry decides to toy with the traitors a bit before he lets them know he's onto them.
He makes up a story about how, yesterday, some drunk guy was talking smack about him in public (which is considered treason). King Henry asks the three traitors if they think he should have mercy on the guy.
Brain Snack: In the 1965 cult classic film The Chimes of Midnight (a.k.a. Falstaff), director Orson Welles turns this anonymous drunk guy into Falstaff and has King Henry pardon him.
Scrope, Cambridge, and Grey are all, "You should punish that guy and make an example out of him!"
Henry says something like "Gee, guys, you really think I shouldn't be merciful to traitors?"
Henry hands the three men some documents to read. (The papers indicate that King Henry has proof of their plot to murder him.)
As the traitors read the incriminating documents, Henry innocently asks why their faces look so pale.
Scrope, Cambridge, and Grey know they're busted – they immediately 'fess up and beg for mercy.
Henry has them arrested and sentences them all to death for high treason.
Cambridge confesses that, even though he accepted French money, he didn't do it for the cash.
Brain Snack: Cambridge supports Edmund Mortimer, who seems to have a better claim to the English throne than Henry V because Mortimer is the great-grandson of Edward III's third son. Henry, on the other hand, is the grandson of Edward III's fourth son. Plus, Henry only inherited the throne after his father (King Henry IV) usurped the crown from Richard II. (In other words, Shakespeare is reminding us that Cambridge's plot isn't so different from what Henry IV did to Richard II.)
As the traitors continue to beg for mercy, Henry refuses and says it's not because he's vengeful – he's just trying to protect England's national security.
The traitors are hauled off to the slammer and Henry takes the opportunity to thank God for revealing the treacherous plot. This, he reasons, must be a sign that God wants him to invade France.