The Hateful Eight-Year-Olds is the twenty-first episode of Season 31.
Lisa makes a new friend who loves horses, but then becomes part of a circle of snooty young rich girls. Meanwhile, Homer takes Marge on a romantic cruise.
Lisa is invited to a birthday party with Addy who loves books and horses like Lisa and when Bart breaks all the horse models she becomes angry at him, and cuts the string of sibling hood. When she arrives at the sleepover, she discovers that not only does Addy have a big house, a party manager but she also has real horses. Addy introduces her to the crew Bella-Ella, Tessa Rose and Sloan all of whom are snooty high class girls and they call Addy, Addison. At the party when Addy opens up her presents they are all rich items. Lisa gives her a toy horse set which she makes fun of, throws away and Lisa gets teased by the girls. She tries to text and call her parents to come get her, who are on a party boat with Weezer, but due to the bad reception she cannot get through to them. She then asks the party planner to drive her home, but she tells Lisa this party has to get good reviews if not she'll have to work at Bubba Gump shrimp. She then brings Lisa back to the party and Lisa tries to get another chance in, the girls decide to watch a mature tv show and when Lisa gets terrified during a scene, the girls humiliate her on social media. Lisa goes into the staff bathroom, when Addy goes to check on her Lisa remarks that the Japanese toilet is the only comforting voice she has heard all evening. Eventually Addison comes clean and tells Lisa the main reason she invited her to the party was because the other girls were so mean to her until now and she then tells Lisa that next time, she'll bring someone else like her to a gathering and then Lisa will be accepted, but Lisa does not like this at all. She finally decides to call Grandpa but Bart picks up who thinks that the string of sibling hood is still cut, but a desperate Lisa begs him to come. The girls take away her phone because she was talking into it. It becomes a game of keep away and ends up on the shelf.
Meanwhile Marge and Homer are enjoying their time on the cruise until the band starts flirting with Marge and Homer stands up to the band. As it turns out the lady behind Marge was the one the singer was flirting with and Marge is embarrassed. Homer then accidentally sends their music off the boat and everyone is mad at Homer. Bart finally arrives for Lisa, who couldn't be happier to see him but before they can go Bart tells Lisa to get revenge on the girls for humiliating her and she gets an idea when the girls are braiding their hair. While they sleep, Lisa coats their braided hair with saddle polish and posts online but her mom calls and asks about all the calls and texts but Lisa says it's not a issue anymore. Suddenly the girls wake up and the braids are so dried, they crack off and their hair looks just like Lisa's. Bart and Lisa head for the stables and there Lisa finds out that Bart is actually afraid of horses but thanks to her he overcomes his fear. Soon enough as they ride away, the girls catch up to them and Lisa wants to talk to her book buddy Addy, but feels she has to be Addison because Addy is a little girl. Lisa is able to get Addy back with a book they read the first time they met. Addy pulls on the three girls vests which inflates giving Lisa and Bart a chance totake off. Addy then ends her friendship with the girls and Bart and Lisa ride home.