The town of Hadleyburg, the “most honest and upright town in all the region around about," is little more than a small village somewhere in an unspecified, but certainly rural, corner of the United States.
In the story, Hadleyburg is as famous for its honesty as for its brutal insularity. The plot of the story is set in motion because the town has a reputation for cruel treatment of outsiders who visit—the Hadleyburg citizens refer to them derisively as "strangers"—and, at some undisclosed point in the past, the town slighted a powerful stranger who takes it upon himself to get revenge on the town by exposing its famous honesty to be little more than a virtuous façade.
Although Twain's story is entirely fictional, and no town named Hadleyburg has ever existed, some scholars have pointed out that Twain himself once experienced a particularly cool reception at a lecture at Oberlin College in (what was then) the small rural town of Oberlin, Ohio. It is possible that Twain meant to correlate Hadleyburg with Oberlin—an important cultural center in the region that Twain may have found to be unwelcoming and insular—and to write his story of Hadleyburg as his own act of vengeance.