henry iv is misnamed since the play isn't really about king henry but about his son, prince hal, and his enemies, especially henry percy (aka 'hotspur') who is a rival to hal. hotspur is one of the leaders of the rebellion against the king and, at a tender age, is already an accomplished soldier. his story provides the drama of the play. hal, on the other hand, has fallen out of favor with the king, and is whiling away his days in the company of dissolute company, led by sir john falstaff, one of shakespeare's great characters. his adventures with sir john provide the comic relief. fortunately for the king, hal sheds his prodigal ways in time to save his father and his crown in the battle at shrewsbury, where, coincidentally, hal meets and slays his rival, hotspur.
this is one of shakespeare's best plays. the story of the rebellion is intriguing, and the adventures of hal and falstaff are laugh-out-loud hilarious. the culmination of the two stories in the final battle scene is wonderful. this is a fitting sequel to richard ii.
note that there are some historical inaccuracies and even outright inventions in this play. foremost is the character of falstaff who is pure invention (and genius). the story of hal's adventures stems from his reputation, enhanced by legend, as a playboy. falstaff was the perfect foil for a carousing prince. the biggest inaccuracy is hotspur's age. he was actually of the generation of henry iv, and not as young as he's depicted in the play. shakespeare made him younger to enhance, maybe even create, the rivalry with hal. there are other inaccuracies here, but better for the reader to consult 'shakespeare's kings', an excellent book by saccio that explains the history of the period and the discrepancies in the play.