The Duke of Gloucester, the King's uncle and arch-enemy, has been murdered. Most people suspect Thomas Mowbray did it at the King's command. Henry Bolingbroke, the King's cousin, has accused Mowbray to his face. Together they are summoned before Richard II.

Henry Bolingbroke, Duke of Lancaster, aided by the Earl of Northumberland and his son Harry Percy deposed the rightful King, Richard II, and Bolingbroke ascend the throne as King Henry IV. Richard was imprisoned and later murdered in Pomfret Castle. Henry, in remorse, vows a pilgrimage to the Holy Land. Northumberland and Hotspur become dissatisfied with the King's lack of favour. Henry is worried about the behaviour of his son, the Prince of Wales.

The Percy family, Henry Bolingbroke's chief allies in deposing and murdering Richard II, have been cheated of their just rewards and now plot to overthrow the King. Worcester, the architect of their rebellion, arranges a meeting in Wales to decide how Great Britain will be divided among them. The King's army, part of it commanded by Prince Hal, marches westward to encounter the rebels. Even Sir John Falstaff receives a commission and marches off to join the battle at Shrewsbury.

The rebellion promoted by the Percy family against their former ally King Henry IV has been heavily crushed at Shrewsbury. The rebels were weakened by the defection of the armies promised them by Northumberland and Owen Glendower, and greatly outnumbered by the King's forces. Their leader, the gallant Hotspur, has been killed in single combat with Prince Hal. Conflicting rumours reach Warkworth in Northumberland, where the Earl is eagerly waiting for news of the battle and of his son.

The burdens of kingship and of failing health weigh heavily upon Henry IV. After his victory at Shrewsbury he has to organise a new campaign against the northern rebels. Disappointed in Prince Hal, who returns to his customary low haunts in London, the King sends his younger son, Prince John of Lancaster, to negotiate with the rebel leaders. Falstaff goes off to Gloucestershire to muster recruits, and there falls in with an acquaintance, a simple country Justice, Master Shallow.