Henry IV is not famous except for the plays that bear his name: He is not even a "much-maligned" king. He is normally overshadowed by his far more famous son, Henry V, despite his many achievements. Shakespeare is partly to blame for Henry's diminution and his son's exaggerated stature but not completely; we just don't know what to make of a man who took the throne from the rightful king.
In this lecture, his reputation is traced from his youth (when he was praised and revered far and wide) to his later years, when he was feared and disliked and seen as a disappointment. The reasons for that decline are explained and a new, more positive view taken of his resilience. In conclusion, one has to see something deeply distinguished in the way he weathered the storm of his later years and prepared the way for his son's martial reign.