As part of a school assignment, Lisa had to look at her family tree for any ancestors of note.  After failing to find anyone but scoundrels, Lisa checks the attic for any history and finds Eliza Simpson's diary.  We switch to flasbacks as Lisa reads the story of her ancestor who helped try to free a slave, Virgil, via the underground railroad.  However, the diary disintegrates just as Eliza is about to get caught with Virgil.  Lisa and Marge find more of the story in the library via a cookbook.   Lisa presents what she has for Black History Month when Milhouse tells her the story actually ends with him getting caught and Eliza giving up.

At the end, it's actually Grandpa that tells the rest of the story.  Mabel (Marge) actually runs off with Virgil, gets him freed, and marries him.  Thus the Simpsons are actually 1/64th Black.