In France (where the rest of the play takes place), King Charles and his son the Dauphin talk about how the English troops are about 2 seconds from knocking on France's front door.
King Charles wants to make plans to defend his kingdom, but his son tells him to chill out and stop being a scaredy cat because England's king is nothing more than a "vain, giddy, shallow, humorous youth."
Charles isn't so sure. He reminds everyone of the time that Henry's great-grandfather (Edward III) and his great uncle (Edward the Black Prince) stormed France and made the French army look like a bunch of chumps. If Henry V is anything like his ancestors, France is in serious trouble.
A Messenger brings word that King Henry's Ambassadors are close by and want a meeting with the French king.
The Duke of Exeter enters with a message from the English monarch. Basically, King Henry says he wants King Charles to step aside peacefully while he helps himself to the French throne, which he's legally entitled to.
King Charles asks "Or else what?"
Exeter says something like "Henry is totally going to invade France like a 'fierce tempest' of 'thunder.' The earth will quake and war will open its jaws and swallow everyone whole."
Then Exeter sweetly adds that King Henry also wanted him to say "hi" to the Dauphin and thank him for the chest of tennis balls.
King Charles says he'll think about it.
His son, the Dauphin, doesn't need to think about anything. He dares Henry to bring it.