After a trip to a petting zoo, Lisa finds herself unable to eat lamb exposing her to ridicule and resentment from her father and friends. But with help from Apu and Paul and Linda McCartney, she makes an effort to stick to her new vegetarian view.
Grampa and the Simpsons visit Storytown Village, which is a boring amusement park for 1-7½ year olds. (Marge had previously said that the trip would be fun for everyone, including Lisa, who is eight.) They also visit the park's petting zoo and are immediately enraptured as they see one little lamb after another, each one more absurdly adorable than the one before it.
Upon returning home, Homer finds that Ned Flanders is throwing a family reunion, and feels upset that Ned didn't invite him ("after I painted those cool stripes all over your car"). In a moment of split-second thought, Homer then decides to throw a barbeque and only invite who he wants. When Ned asks if he can come, Homer agrees... before suddenly realizing why he was throwing the barbeque in the first place, and exclaims his famous catchphrase, "D'oh!"
That night, Marge serves lamb chops for dinner, and Lisa cannot eat it. She keeps hearing the bleating voice of the lambs at the petting zoo - "Li-i-i-sa, I thought you lo-o-o-ved me-e-eee!" Lisa pushes her plate away, crying, "I can't eat this. I can't eat a poor little lamb!" Naturally, Homer notes the distinction between lamb, and a lamb. Her mother, trying to help, offers up rump roast, chicken breast, and hot dogs instead, but Lisa suddenly makes the connection between these dishes and their living counterparts. "No, I can't! I can't eat any of them!"
At school, her newfound vegetarianism becomes a problem when she is almost forced to dissect a live worm who she imagines pleading, "Li-i-isa, what did I ever do to you-u-u-u?" She's a little confused ("Why does it talk like a lamb?") but no less committed to her cause, and so she refuses to dissect it. At lunch, the only vegetarian option is a hot dog bun, which Lunchlady Doris proclaims "rich in bunly goodness". (In the Spanish dubbed version, Doris claims it's "rich in wheat and mustard".) Her caustic questioning of the exact point when Doris lost her enthusiasm for her job results in the second secret 'independent thought alarm' she's triggered that day, prompting Principal Skinner to think that the students have become overstimulated. He orders Groundskeeper Willie to h have all colored chalk removed from the classrooms. The Scotsman furiously rants that he warned them before because, in his opinion, the chalk had been "forged by Lucifer himself!"
Skinner then talks to Lisa's class that there's an independent thinker (whom he will not reveal, but unknowingly does so) in the class and shows them a (rather dated) video of man and a boy looking at cows getting killed and proposing the idea that "cows might eat us if we don't eat them". Homer then shows off his pig-shaped invitations, which advertise his "BBBQ," with the extra "B" for "BYOBB." When Bart asks what the extra "B" is for, Homer says, "That's a typo." At home her newfound vegetarian stance is not tolerated or understood very well either, with Homer and Bart both mocking her for rejecting meat. Homer and Bart even start a dance line, singing "You don't win friends with salad," and even Marge joins in when Lisa requests that something other than the traditional meat be served at Homer's upcoming barbecue.
Meanwhile, Homer hosts his barbecue complete with roast pig. Lisa - who is becoming increasingly self-righteous regarding her vegetarianism - brings gazpacho but is laughed out of the yard and into her room, where she sulks on her bed and resents the partygoers for rubbing their carnivorous habits "in [her] face". Just then, a hamburger haphazardly flipped by Homer for Bart flies through her window and lands on her face.
Enraged, Lisa climbs aboard a riding mower, and drives away with the roast pig in tow with Marge shouting at her to stop (although she erroneously refers to her as Bart initially until Bart asks what he did wrong, causing her to remember and apologize for the mistake, mentioning that it was a force of habit before correctly telling Lisa to stop). Homer and Bart chase after her, but she pushes the pig off a slope and they're too late. The pig rolls through bushes, into the river, and is shot into the air by a hydroelectric dam's suction.
Meanwhile, Mr. Burns shocks Smithers by saying that he will give a million dollars to a local charity...when pigs fly. Just then the roast pig flies into view. Burns is utterly shocked, but still refuses to donate the money.
Back at home, Homer scolds Lisa for ruining his barbecue and sends her to her room, but she rebukes him and says he was wrong for serving and eating meat. Lisa and Homer are too angry to speak to one another. They fight and she leaves the house. While walking, she is mocked by classmates and hit with a barrage of bad meat-related advertisements (Sherri and Terri taunt, "Look at Mrs. Potato Head! She has a head made out of lettuce!", giggling). The pressures to conform to an omnivorous society finally become too great, prompting Lisa to grab a hot dog off of the grill at the Kwik-E-Mart, take a bite, and shout, "There! Is everybody happy now?"
However, Apu - himself a vegan (though he erroneously states he is a vegetarian on the show) - reveals that she has, in fact, eaten a tofu dog, and takes her through a secret passageway to the garden on the Kwik-E-Mart roof to meet guest stars Paul and Linda McCartney. One brief heart-to-heart later, Lisa is committed once more to vegetarianism. She also learns an important lesson from Apu: "I learned long ago, Lisa, to tolerate others rather than forcing my beliefs on them. You know you can influence people without badgering them always."
Thus inspired, she returns home to make up with her father, and finds him in the street shouting her name, exposing his fear of being viewed as a bad father. Lisa apologizes to Homer, admitting she had no right to ruin his barbecue; he forgives her and offers her "a piggyback--I mean, a veggieback ride."
MSNBC listed the episode as their second favorite in the show. They called the “You don’t win friends with salad” song "one of those archetypal “Simpsons” moments, one in which the writers hit a joke so long that it goes from funny to unfunny and back to funny again."