As Bart watches an Itchy & Scratchy episode, Marge makes a cake for a block party, but when the cake batter splatters everywhere in the kitchen, Marge is forced to get a store bought cake. At the block party, Marge places her store bought cake on the table at the same time a new neighbor places his store bought cake. Despite her initial anxiety, Marge feels at ease meeting Booth Wilkes John, a British man, who invites Marge over to adult game night with his wife. Even though Marge wants to go, she thinks back to when Homer ruined adult game night for her. Marge declines at first citing church the next day, but Reverend Lovejoy says he's moving service to Monday. Marge talks to Homer in bed about not having any friends, so she wants to go to this game night, yet Lisa chimes in with being okay with not having friends herself (which makes Marge extremely uncomfortable). Before game night, Marge reminds Homer of the proper etiquette.
At the new neighbor's home, Marge and Homer mingle with the guests. The game begins with role play based on Clue, and Homer unknowingly reveals the murderer in the story, prompting the host to kick the Simpsons out. Marge feels depressed over not having friends, but she's set on not having her daughter go through life without friends, so Marge decides to have a party for Lisa and invite everyone who knows her. The only problem is that only one boy shows up. Marge decides to scrap the idea, but the one boy won't leave unless he gets a party. Marge and Homer throw a party for the kid and send him off with party favors just in time as Lisa arrives home. At Springfield Elementary, Marge goes on a mission to figure out why Lisa does not have any friends. Miss Hoover spells it out very clearly to Marge that no one in the class likes her for being too isolated and unique. She also points out that Lisa is too insistent in forcing her classmates to accept her views and doesn't care to respect theirs. Marge refuses to accept Miss Hoover's answer and tries to find someone who does like her. When the class has to practice square dancing, Lisa is left without a partner until Tumi, an outside student, steps in to save the day.
Lisa's new friend, Tumi, has the same interests as her, but Bart is clearly suspicious and believes Lisa is being duped. When Bart follows her to the Krusty Burger play area, he learns the truth when he witnesses Marge handing over some cash to the girl. Bart delivers the photographic evidence to Lisa, who confronts Marge. Lisa tells her mother that she would have found a friend in a decade, if she waited. When Marge tries to apologize for her actions, Lisa refuses to forgive her and retreats in her room. Marge mopes around and with Homer and Grampa. Grampa reveals that he paid Lenny and Carl to befriend Homer in the sandbox and to this day they are still friends, much to Homer's dismay. Homer asks if that story is true to which Grampa replies that he wouldn't pay ten cents to prevent a lion from eating him. Meanwhile, Marge has a heart-to-heart conversation with Lisa and tries to explain her reasons behind it. She still refuses to forgive Marge for her evil deeds and intends to tell every psychiatrist about it. Her mother is in tears, which makes Lisa secretly proud and plans to use her new-found tactic to get what she wants from Marge. Eventually, the thought of Marge in tears makes Lisa apologize and wishes her a Happy Mother's Day. Homer and Bart make a getaway, clearly aghast that they have forgotten Mother's Day.
The next day, Tumi apologizes to Lisa for her behavior and why she accepted the bribe for payment on a new belt. She also admits she does like being friends with Lisa. Lisa forgives her, but asks for her to be more honest with her from now on. However, Lisa ditches her after she finds out she eats horse.