"Lisa's Substitute" is the nineteenth episode of Season 2. Dustin Hoffman–using the pseudonym Sam Etic–guest starred in it as Mr. Bergstrom. This episode is notable for being the first full appearance of Ralph Wiggum.
When Lisa's teacher Miss Hoover gets Lyme Disease, Mr. Bergstrom takes over the class. Lisa finds Mr. Bergstrom's teaching methods incredibly inspiring and discovers an entirely new love for learning, but when Mr. Bergstrom leaves, Lisa doubts that anyone else in her life (including her own father) can be the man that Mr. Bergstrom was. Meanwhile, Bart runs against Martin Prince for class president.
Lisa's teacher, Miss Hoover, thinks she has come down with Lyme disease and is replaced by substitute teacher Mr. Bergstrom. Because of his unorthodox teaching methods, Lisa takes a liking to him, perhaps even getting a crush of sorts.
Bart's class, meanwhile, prepares to elect a class president. Mrs. Krabappel nominates Martin, while Sherri and Terri nominate Bart. During a debate with Martin, Bart tells jokes that win the class over. Ms. Krabappel embarrassed by his shenanigans, quickly writes a note to Skinner for Bart to give. Later on, Marge suggests to Homer to spend more time with Lisa at a museum. Only problem is that he wants to spend his free time helping Bart with his campaign. Marge then puts her foot down and forces Homer to go regardless.
Lisa runs into Mr. Bergstrom at a museum and is embarrassed when Homer displays his ignorance. Sensing that Lisa is missing something in her relationship with her father, Mr. Bergstrom takes Homer aside to suggest he be a more positive role model. Bergstrom also admits he sees Homer as a negligent father towards his daughter. After venting to her mother about Homer ruining her 'one chance' to get to know Bergstrom outside of the classroom, Lisa is given permission to invite him to family dinner, only to be shattered when she finds that Miss Hoover is back and Mr. Bergstrom is gone. She rushes to his apartment and finds him having left for another job. She rushes to the train station to catch him, and confesses to him that she will be lost without him. To comfort her, he writes her a note and tells her that if she ever feels alone and like she can't rely on anybody, its contents are all she needs to know. He then boards the train and departs. The note reads, "You are Lisa Simpson." Meanwhile, certain of Bart's inevitable victory thanks to his popular campaigning, none of the children in his class–including himself–actually voted, giving Martin the victory with just two votes: one from himself, and the other from his best friend, Wendell Borton.
Devastated by Mr. Bergstrom's departure, Lisa takes her grief out on Homer, calling him a baboon. Marge tells Homer to console Lisa, explaining how their daughter is very hurt emotionally and is in need of her father. Homer enters Lisa's room and finds her crying over her desk. He is uncertain of how to deal with Lisa's sadness, and is uncomfortable seeing his daughter crying. Homer explains to Lisa how he can't really understand how it feels to lose someone important: everyone he has ever loved and cared about still lives with him. He then alludes to Lisa calling him a baboon, and in a loving manner mimics a monkey, cheering her up.
Lisa apologizes to Homer for calling him a baboon, and he accepts the apology. Finding Bart still seething over the election result, Homer cheers him up by reminding him that all the job of class president would have really meant was a lot of extra work with little reward, making Bart feel happy that he lost the election. Finally going by Maggie's room, he places her pacifier in her mouth. Proud that he helped all three of his children, Homer goes to bed with Marge happily that night, stating he is "on the biggest roll" of his lifetime.