Henry Bolingbroke—now King Henry IV—is having an unquiet reign. His personal disquiet at the murder of his predecessor Richard II would be solved by a crusade to the Holy Land, but broils on his borders with Scotland and Wales prevent that. Moreover, he is increasingly at odds with the Percy family, who helped him to his throne, and Edmund Mortimer, the Earl of March, Richard II's chosen heir.
Adding to King Henry's troubles is the behaviour of his son and heir, the Prince of Wales. Hal (the future Henry V) has forsaken the Royal Court to waste his time in taverns with low companions. This makes him an object of scorn to the nobles and calls into question his royal worthiness. Hal's chief friend and foil in living the low life is Sir John Falstaff. Fat, old, drunk, and corrupt as he is, he has a charisma and a zest for life that captivates the Prince.
The play features three groups of characters that interact slightly at first, and then come together in the Battle of Shrewsbury, where the success of the rebellion will be decided. First there is King Henry himself and his immediate council. He is the engine of the play, but usually in the background. Next there is the group of rebels, energetically embodied in Henry Percy ("Hotspur") and including his father, the Earl of Northumberland and led by his uncle Thomas Percy, Earl of Worcester. The Scottish Earl of Douglas, Edmund Mortimer and the Welshman Owen Glendower also join. Finally, at the center of the play are the young Prince Hal and his companions Falstaff, Poins, Bardolph, and Peto. Streetwise and pound-foolish, these rogues manage to paint over this grim history in the colours of comedy.
As the play opens, the king is angry with Hotspur for refusing him most of the prisoners taken in a recent action against the Scots at Holmedon. Hotspur, for his part, would have the king ransom Edmund Mortimer (his wife's brother) from Owen Glendower, the Welshman who holds him. Henry refuses, berates Mortimer's loyalty, and treats the Percys with threats and rudeness. Stung and alarmed by Henry's dangerous and peremptory way with them, they proceed to make common cause with the Welsh and Scots, intending to depose "this ingrate and cankered Bolingbroke." By Act II, rebellion is brewing.
Meanwhile, Henry's son Hal is joking, drinking, and thieving with Falstaff and his associates. He likes Falstaff but makes no pretense at being like him. He enjoys insulting his dissolute friend and makes sport of him by joining in Poins’ plot to disguise themselves and rob and terrify Falstaff and three friends of loot they have stolen in a highway robbery, purely for the fun of watching Falstaff lie about it later, after which Hal returns the stolen money. Rather early in the play, in fact, Hal informs us that his riotous time will soon come to a close, and he will re-assume his rightful high place in affairs by showing himself worthy to his father and others through some (unspecified) noble exploits. Hal believes that this sudden change of manner will amount to a greater reward and acknowledgment of prince-ship, and in turn earn him respect from the members of the court.
The revolt of Mortimer and the Percys very quickly gives him his chance to do just that. The high and the low come together when the Prince makes up with his father and is given a high command. He vows to fight and kill the rebel Hotspur, and orders Falstaff (who is, after all, a knight) to take charge of a group of foot soldiers and proceed to the battle site at Shrewsbury.
The battle is crucial because if the rebels even achieve a standoff their cause gains greatly, as they have other powers awaiting under Northumberland, Glendower, Mortimer, and the Archbishop of York. Henry needs a decisive victory here. He outnumbers the rebels, but Hotspur, with the wild hope of despair, leads his troops into battle. The day wears on, the issue still in doubt, the king harried by the wild Scot Douglas, when Prince Hal and Hotspur, the two Harrys that cannot share one land, meet. Finally they will fight – for glory, for their lives, and for the kingdom. No longer a tavern brawler but a warrior, the future king prevails, ultimately killing Hotspur in single combat.
On the way to this climax, we are treated to Falstaff, who has "misused the King's press damnably", not only by taking money from able-bodied men who wished to evade service but by keeping the wages of the poor souls he brought instead who were killed in battle ("food for powder, food for powder"). Now on his own Falstaff is attacked by the Douglas during Hal's battle with Hotspur, but plays possum and is presumed dead. After Hal leaves Hotspur's body on the field, Falstaff revives in a mock miracle. Seeing he is alone, he stabs Hotspur's corpse in the thigh and claims credit for the kill. Though Hal knows better, he allows Falstaff his disreputable tricks. Soon after being given grace by Hal, Falstaff states that he wants to amend his life and begin "to live cleanly as a nobleman should do".
The play ends at Shrewsbury, after the battle. The death of Hotspur has taken the heart out of the rebels, and the king's forces prevail. Henry is pleased with the outcome, not least because it gives him a chance to execute Thomas Percy, the Earl of Worcester, one of his chief enemies (though previously one of his greatest friends). Meanwhile Hal shows off his kingly mercy in praise of valor; having taken the valiant Douglas prisoner, Hal orders his enemy released without ransom. But the war goes on; now the king's forces must deal with the Archbishop of York, who has joined with Northumberland, and with the forces of Mortimer and Glendower. This unsettled ending sets the stage for Henry IV, Part 2.