Set in a kingdom plagued with rebellion, treachery, and shifting alliances in the period following the deposition of King Richard II, the two parts of Henry IV focus especially on the development of Prince Hal (later Henry V) from wastrel to ruler rather than on the title character. Indeed, the king is often overshadowed not only by his son but also by Hotspur, the young rebel military leader, and by Hal's roguish companion Falstaff. Secondary characters (many of them comic) are numerous. The plot shifts rapidly between scenes of raucous comedy and the war against the alliance of the Welsh and the rebellious Percy family of Northumberland.

As Part 1 begins, Henry IV, wearied from the strife that has accompanied his accession to the throne, is renewing his earlier vow to make a pilgrimage to the Holy Land. He learns that Owen Glendower, the Welsh chieftain, has captured Edmund Mortimer, the earl of March, and that Henry Percy, known as Hotspur, son of the earl of Northumberland, has refused to release his Scottish prisoners until the king has ransomed Mortimer. Henry laments that his own son is not like the fearless Hotspur. As the war escalates, Glendower, Mortimer (now married to Glendower's daughter), and Hotspur (now allied with the Welsh) conspire to divide Henry's kingdom into three equal parts.

Meanwhile, Prince Hal and his cronies, including the fat, boisterous Falstaff and his red-nosed sidekick, Bardolph, have been drinking and playing childish pranks at Mistress Quickly's inn at Eastcheap. Hal, who admits in an aside that he is consorting with these thieving rogues only temporarily, nevertheless agrees to take part with them in an actual highway robbery. He does so under certain conditions: the money is to be taken away from Falstaff and his companions by Prince Hal and his comrade Poins in disguise, and the money is then to be returned to its rightful owners, so that the whole caper is a practical joke on Falstaff rather than a robbery. This merriment is interrupted by Hal's being called to his father's aid in the war against the Welsh and the Percys. Hal and his father manage to make up their differences, at least for a time, most of all when Hal saves the life of his father in combat. Hal further proves his valour in battle, where he chides Falstaff for malingering and drunkenness and then kills Hotspur in personal combat during the Battle of Shrewsbury. Hal laments the wasteful death of his noble opponent and of Falstaff, on the ground nearby. But Falstaff was only feigning death, and, when he claims to have killed Hotspur, Hal agrees to support the lie. At the play's end, rebellion has been only temporarily defeated.

For a discussion of this play within the context of Shakespeare's entire corpus, see William Shakespeare: Shakespeare's plays and poems (Shakespeare, William).