The main plot of Henry IV, Part 1 is about the rebellion of the Percies, the northern baronial family who had helped Henry depose Richard II and become king. They are joined by the Scottish Earl of Douglas, Edmund Mortimer, Earl of March, claimant to the throne, and Owen Glendower, a Welsh noble.
Henry is presented first as a ruler who has been beset with troubles from the start: civil unrest in England, attacks by Scottish forces moving across the northern border, and the defeat and capture of the still-loyal Mortimer by Glendower. He thus is unable to fulfill his earlier vow to lead a crusade to the Holy Land. But there is one piece of good news: English forces led by young Hotspur have defeated the Scots at Holmedon and have captured the renowned Earl of Douglas. Yet this especially gives the harassed king reason to lament the dereliction of his son and heir, Prince Henry, who persistently has avoided the court and public responsibility and spends his time in the company of the elderly, high-spirited Sir John Falstaff, as well as the lowly patrons of the Boar's-Head Tavern in Eastcheap.
The comic subplot deals most amusingly with this same Falstaff and his companions, including Prince Hal, as he is appropriately called in this setting. In the initial episode, Hal joins with Poins, Bardolph, and Peto in a plan to jokingly deceive Falstaff, contriving to have him participate in a robbery at Gadshill, be robbed in turn, and finally exposed as a coward and liar. Alone, Prince Hal soliloquizes, letting the audience know that, although he now chooses to enjoy himself in riotous company, he has no illusions about the character of his associates and will redeem himself publicly at the proper time.
The main plot takes precedence at this point. Once more at the court, King Henry confronts the Percies-Northumberland, Worcester, and Hotspur. He sternly scolds them and especially demands to be told why the ranking Scottish prisoners taken by Hotspur have not been turned over to the Crown. The Percies, deeply resenting the fact that the man they helped to the throne now intends to enforce absolute obedience, begin to plot their revolt. They will make peace with Glendower and gain his support and that of Mortimer. Then, aided by Welsh and Scottish forces, the latter led by Douglas, they will war against the usurper, King Henry IV.
The comic subplot now gains center stage. Arrangements are completed for the robbery at Gadshill during the night. When the victims, a group of travelers, approach, Poins and the prince use an excuse to separate themselves from Falstaff and the others. Falstaff, forced to proceed on foot, is the leader of the robbers, who then are set upon by the disguised prince and Poins and robbed in turn. Falstaff quickly beats a retreat.
Again the action shifts to the main plot. Hotspur reads a letter from an English noble whose aid he has solicited. He can hardly contain himself as he reads the excuses offered, and he denounces the writer as a coward. Then, in an exchange with his attractive young wife, Kate, he reveals himself as a man practically obsessed with matters relating solely to the planned revolt against the Crown.
In Eastcheap at the Boar's-Head Tavern, Falstaff is exposed as the dupe who has been tricked by the prince and Poins. But, not unexpectedly, in view of his already demonstrated wit, he not only survives the ordeal of being laughed at as a coward and liar but emerges comically triumphant. Prior to all this, Hal had made satiric reference to the ability of Hotspur; now an emissary from the king informs Hal that he must appear at court promptly. This turn of events provides the prince the opportunity to participate with Falstaff in a "play extempore," each successively enacting the role of King Henry rebuking the wayward son and heir.
At the residence of the Archdeacon in North Wales, Hotspur, Worcester, Mortimer, and Glendower hold a parley, one not without some disagreement. They plan the campaign against the royal forces and the later division of England into three parts. The absence of the Earl of Northumberland is significant in light of later events.
In the meantime, the confrontation between the king and Prince Hal takes place, the former sternly rebuking his son and comparing him unfavorably to the valiant Hotspur. But when Hal soberly vows to redeem his tarnished reputation at Hotspur's expense, the king not only forgives him but places him in command of royal forces. At that very moment the two learn that the rebels will be assembled at Shrewsbury.
Once more the action shifts to the Boar's-Head Tavern, where Falstaff is flourishing in the company of Bardolph and Mistress Quickly. The prince and Peto arrive, and we learn that Hal has reimbursed the travelers who had been robbed at Gadshill and has arranged for Falstaff's commissioning as a leader of the king's forces.
Hotspur, Worcester, and Douglas learn that the Earl of Northumberland and his retainers will not join them. Apparently illness has incapacitated him. But Hotspur is not upset by this bad news; sure of victory, he sees this as providing a greater opportunity to impress the multitude, whose support the rebels must gain and retain. Vernon arrives with the news that the royal army has set forth for Shrewsbury and that second in command to the king himself is the Prince of Wales. Hotspur, at first depressed by Vernon's glowing description of Hal, quickly recovers himself and declares that he will fight the prince in single combat. Yet a third report is received: Glendower will require fourteen days to raise a force of Welshmen. But nothing can dampen the eagerness nor restrain the impulsiveness of Hotspur.
Next we see Falstaff as a military commander. He has managed to enlist a group of pitiful, physically unfit "soldiers" whom he will lead to Shrewsbury. Both Prince Hal and Westmoreland remark on their unfitness but do nothing to stop Falstaff.
Sir Walter Blunt, emissary from King Henry, arrives in the rebel camp and addresses the rebel leaders, voicing the conventional condemnation of their disloyalty and conveying to them the king's willingness to listen to their grievances. Hotspur restates the rebels' arguments, and the royal offer is rejected.
In Yorkshire, the Archbishop of York and one Sir Michael, both sympathetic to the rebellious Percies, discuss the upcoming battle at Shrewsbury. Learning that Mortimer and his battle forces will not be able to join the rebels, the archbishop expresses his deep concern for the success of the enterprise. He directs Sir Michael to leave at once to enlist support because he knows that King Henry, aware of his disloyalty, will move against him "if Lord Percy thrive not."
The action now moves to the king's camp at Shrewsbury. There, Worcester and Vernon meet with Henry IV, are given a chance once more to voice their grievances, are lectured on the subject of loyalty, and are offered generous terms if they will disband their forces. Prince Hal speaks words of praise for Hotspur, modestly concedes that he himself has been derelict, and offers to fight his rival in single combat. The two rebel leaders depart, supposedly to report to Hotspur what has been said by the king and prince. Present during all this is Sir John Falstaff, who, left to himself, soliloquizes on the impractical aspects of heroism and honor.
Worcester does not tell his nephew about the king's offer, convinced that under any circumstances the older leaders of the revolt will be the objects of Henry's wrath. But Vernon reports honestly and clearly to Hotspur how Prince Hal conducted himself. When a messenger announces the approach of the royal forces, Hotspur sounds the call to battle.
In the course of that battle, Douglas kills Blunt, mistaking him for the king, and he exchanges words of warlike determination with Hotspur. A contrast is provided by Falstaff, who suddenly appears after the two have left. We learn that he has committed his tattered troops to battle, wherein they, with the exception of one or two, have been slaughtered. When the prince appears, deadly serious, Falstaff employs his verbal wit once more and is sternly rebuked.
Prince Hal rescues his father from the sword of Douglas and meets and slays his great rival, Hotspur. After the prince's departure for another part of the battlefield, Falstaff reappears. Then, when Hal returns with his brother, Prince John of Lancaster, Falstaff claims to have killed the young rebel leader. Neither of the princes bothers to refute him at any length. Worcester and Vernon are captured and later put to death. But Douglas, also a captive, is set free by the generous Prince Hal. The rebel forces have been badly defeated. King Henry then dispatches John of Lancaster to the North, where he will oppose Northumberland and the Archbishop Scroop; he himself will leave with Prince Hal to fight the forces led by Glendower and Mortimer.