You know the answer to this one. The play is so named because it's the first part of two plays about events that go down during the reign of King Henry IV.
Fun Fact: The publisher of the second 1598 quarto added a little somethin' somethin' to the title. It reads like so:
With the battell at Shrewsburie,
betweene the King and Lord
Henry Percy, surnamed
Henrie Hotspur of
With the humorous conceits of Sir
Check it out at the original title page here.
What does this tell us? First, it seems the initial publisher thought the battle at Shrewsbury, Hotspur (Henry Percy), and the antics of Falstaff were the coolest and most important parts of the play. Second, Prince Hal doesn't get any billing here (which isn't so surprising given the fact that critics and audience members really didn't sit up and pay attention to Prince Hal's "coming of age" story until the twentieth century). But don't go feeling sorry for Prince Hal, because he gets a play named after him later on. (Psst. It's called Henry V.) Next, we notice there's no mention of the play being "Part 1," which brings us to our next question. Did Shakespeare intend the play to be a Part 1 of 2 in the beginning?
Our answer: Your guess is as good as ours. Here's a list theories that literary critics are partial to:
Option 1: Big Willy only planned to write one play but it was so successful that his official fan club lobbied for the Bard to write a sequel. He happily obliged, but had to take out a restraining order against a deranged fan who followed him around London screaming things like "Bring back Falstaff!" and "Long live Plump Jack!"
Option 2: One day, Shakespeare was at his desk with his favorite ink-quill, happily scribbling about the reign of Henry IV. Then, uh oh, he ran out of room in his five act play so he had to stop at the Battle at Shrewsbury and start a new play called Henry IV Part 2.
Option 3: Please. Shakespeare planned to write 2 parts all along.
Take your pick and be sure to let us know if you come up with any better ideas.