While working on a geneology assignment, Lisa discovers a diary written by her ancestor, Eliza Simpson. She reads it, hoping to discover an Simpson in her family tree that was not an alcoholic or criminal. She discovers through reading the diary that Eliza and her mother Mabel were part of the underground railroad. Eliza sneaks into a ball hosted by Colonel Burns to meet a slave named Virgil, but as the two make their escape, they are spotted by a mounted patrol. Unfortunately, the diary is too disintegrated for Lisa to read on, and she cannot bear not knowing if Virgil escaped.
Lisa and Marge discover a cookbook at the local library written by Mabel. In it is an anecdote that tells how Eliza and Virgil evaded capture by donning disguises at a traveling circus. They make it back to the Simpson household, but Eliza's father Hiram is suspicious of Virgil's presence. Virgil makes him 'wheel cakes' and Hiram swears to keep Virgil's whereabouts a secret.
Lisa believes Eliza to be a hero for helping Virgil escape, and tells this story at a Black History Month presentation at Springfield Elementary. Milhouse, however, challenges her story and suggests that Eliza was a coward. He reads from the journal of his ancestor, Milford Van Houten, who witnessed Colonel Burns bribe Hiram with a new pair of shoes in exchange for giving up Virgil. Eliza does not stand up against the Colonel and Lisa is crushed to think her ancestor was a coward.
Homer convinces Grandpa Simpson to tell the rest of story to raise Lisa's spirits. He tells of how Mabel threatened to castrate the Colonel with a shotgun and then escaped with Virgil to Canada. There, she divorced Hiram and married Virgil. Grandpa reveals that Virgil and Mabel's son was his great-grandfather, and therefore the Simpsons are 1/64 African American. When questioned why he kept this secret, he notes that his generation was racist. Marge notes that her father was French, to which Homer attributes his own drinking.