The Englishman Gower tells the Welshman Fluellen that he is wanted by the Duke of Gloucester at the "mines," which were tunnels dug under besieged fortifications, so that explosives could be placed there. Fluellen protests that the mines have not been built properly, and anyway the French have dug counter-mines four yards under the English ones, so they may well get blown up.

When Fluellen learns that that it is an Irishman, Captain Macmorris, who is in charge of the siege, he insults him, saying he is ignorant about war.

Macmorris and the Scots Captain Jamy enter. Fluellen approves of Jamy, because he thinks he is knowledgeable about war.

Macmorris bemoans the fact that he has made a mess of the work he was in charge of. Fluellen, wanting to prove how ignorant Macmorris is, tries to engage him in a discussion about the ancient Roman wars. But Macmorris shrugs him off, saying it is no time to talk; battle is being joined and they must make their contributions rather than sit around talking. The two men then quarrel after Fluellen implies that the Irish are a nation separate from the Welsh, a suggestion that angers the Irishman.

The quarrel is interrupted when they hear the news that the town of Harfleur has requested to talk with the besiegers.

Henry enters, before the gates of Harfleur, and demands to know what the governor of the city has decided. He says that if he has to resume the bombardment of the city again, he will reduce Harfleur to ashes and will show no mercy to its inhabitants. He will not be able to restrain his soldiers from rape and pillage. If this happens, it will be the fault of the people of Harfleur, not the English. In light of this, he asks Harfleur to submit, while he still has control of his men. Then he gives an even more bloodthirsty picture of what will happen to man, woman and child in Harfleur if he is denied.

The Governor informs Henry that the Dauphin, to whom they appealed, is not ready to come to their aid. Therefore he submits to their demands.

Henry gives instructions to Exeter to enter the town and fortify it, and treat the citizens well. The following day, Henry will retire with the rest of his forces to Calais (an English possession).

Analysis

Although Fluellen and MacMorris do quarrel, the intention of this scene is to show that all four nations that make  up the British Isles-Scotland, Wales, Ireland and England-contribute to the war. During the fifteenth century, England was more often in conflict with these nations than in partnership with them, so this is an illustration of Henry's skill in bringing rivals together for a common venture.

Henry's speech to the governor of Harfleur is bloodcurdling in its ferocity, but the likelihood is that Shakespeare is once more demonstrating the King's skill at getting the outcome he wants. The mere threat of atrocities is enough to compel the town to submit. It is notable that when Henry does gain entrance to the city, he instructs his men to behave in a decent fashion. Some modern readers, however, have censured Henry because of his habit of blaming the other side for the mayhem he is ready to commit. He does this at Harfleur, just as he had told King Charles that he, Charles, would be to blame for the devastation that a war would cause.