As Springfield's bicentennial celebration approaches, Lisa discovers the real truth behind town founder, Jebediah Springfield and finds herself at odds with a protective museum curator who wants to keep Jebediah's unattractive past a secret. Meanwhile, Homer becomes obsessed with being the official town crier for the bicentennial celebration.
Springfield's bicentennial is coming up, to celebrate, the town prepares a parade. Lisa, decides to enter a report about Jebediah Springfield into a school contest. To research for the report, she visits the history museum, where she meets Hollis Hurlbut, a renowned scholar who specializes in researching Jebediah Springfield. Meanwhile, Homer tries out for the position of town crier in the upcoming parade. Flanders, who was originally town crier, gracefully gives up the position and Homer takes over.
Lisa finds a rolled up piece of parchment concealed in Jebediah's flute. The parchment reveals that Jebediah Springfield's real name was Hans Sprungfeld, and that he was a murderous pirate who shot a wild buffalo rather than taming it, and got in a fistfight with George Washington. Lisa reveals this to Homer and they set off to reveal the truth, largely because Homer believes Lisa has been right about things in the past. No one believes them, so they set out to dig up Jebediah's corpse: Hans Sprungfeld had a silver prosthetic tongue, so if they find the tongue, they prove that Jebediah Springfield was not the hero he was thought to be. When they dig up the corpse, they find no tongue.
Homer is banned from the parade, and he and Lisa are shunned. Lisa gives up until the middle of the night when she is visited by the ghost of George Washington, who commends her for exposing Hans Sprungfeld as a fraud and hints to look to the famous torn portrait of himself; the missing piece being where Sprungfeld wrote his confession. Determined to find the tongue, Lisa goes back to the museum. There she finds that Hollis Hurlbut stole and hid the tongue, in order to shield the town from the truth when he found out he'd dedicated his life to a criminal.
Hurlbut finally accepts that Jebediah Springfield was really a brutal pirate, and says they must stop this sham celebration. However, just as Lisa is about to reveal the truth about Hans Sprungfeld to the town, she sees positive factors such as veterans attending the parade and kids on floats, to which she simply says Jebediah Springfield was a great man. When Hurlbut asks Lisa in private why she did not tell the town about Hans Sprungfeld, she says the myth about Jebediah Springfield is valuable due to it bringing out the best in the community. To reward Homer for believing in her earlier, she sits atop his shoulders as he participates in the parade.