"Whacking Day" is the twentieth episode of Season 4.
After ruining a review by Superintendent Chalmers of Springfield Elementary by commandeering on Groundskeeper Willie's tractor while trying to escape from detention by Principal Skinner for the review, Principal Skinner is furious with Bart and expels him, forcing Marge to home school Bart. Meanwhile, everyone in Springfield is excited for Whacking Day, but as the holiday approaches, Lisa condemns it as violent. Whacking Day is a decades-long tradition which calls for the townspeople to beat snakes to death with sticks. Lisa plans to save the snakes with the help of Whacking Day's honorary MC, Barry White, who uses his sonorous bass vocal style to lure the snakes to safety, where no one can whack them.
Principal Skinner lures Bart, Jimbo, Kearney, Dolph and Nelson to the school basement with the promise of free mountain bikes. When they get there, however, the boys informed that they are being "swept under the rug" during an inspection by Superintendent Chalmers. Bart escapes through a ventilation shaft, to get outside and unlock the door for the rest of the bullies. But before he reaches the basement, he observes Groundskeeper Willie's tractor with the keys in the ignition and (after having mental conversation with said tractor) takes it for a joyride. He quickly loses control, bails out and the tractor strikes Chalmers at full speed in the rear. While being treated for the injury, Chalmers informs Skinner the accident has cost him a promotion as the Superintendent. Skinner tries to change his mind but is silenced. Skinner then takes a deep breath of unfathomable rage, turns around and calmly tells Bart that there won’t be a detention this time, because he's officially expelled him from Springfield Elementary.
Meanwhile, Kent Brockman announces that a celebration that was found to be "disgusting and puerile" by a group of hillbillies is coming up. The upcoming celebration is Whacking Day - every May 10, the people of Springfield drive snakes to the center of town and beat them to death. Lisa is appalled at the upcoming Whacking Day celebration, but her protests fall on deaf ears. She tries to appeal to Homer to not lower himself to the level of others, but Homer is set on participating in Whacking Day.
Bart remains expelled from Springfield Elementary for striking superintendent Chalmers, and his parents try enrolling him in a private Christian school. He gets in trouble and is expelled almost immediately for singing "Beans, Beans, the Musical Fruit" when asked for a psalm, and is chased from the building by the teacher and other students. Marge then decides to homeschool Bart. For one of her assignments, Marge gives Bart "Johnny Tremain" to read, which was a favorite of hers when she was a little girl. Bart gains interest in the book when learning the protagonist was deformed by being wounded in a war, and afterwards starts becoming a serious student, even missing an episode of Itchy and Scratchy to read the book. Due to his vast interest in the book, Marge takes Bart on a "field trip" to Olde Springfield Towne. While there, their tour guide (using a slide show) shows a photo of Jebediah Springfield leading an attack of Fort Ticonderoga and then creating Whacking Day on May 10, 1775. However, Bart quickly realizes Jebediah was leading the assault on Fort Ticonderoga the day he supposedly created Whacking Day and questioned how he could have been in two places at the same time. At a loss for words, the Squeaky-Voiced Teen in charge of the tour calls security to have Bart and Marge ejected.
Barry White (who Mayor Quimby wrongly introduces as Larry White) arrives to begin festivities, he originally says he's honored, but when he discovers what the holiday is about, he is disgusted and exclaims, "You people make me sick!" to the ignorant joy of the assembled crowd, and replaced by Miss Springfield. Bart finds out about the true origin of Whacking Day. He suggests to Lisa that they can lure the snakes to safety by playing music with a lot of bass and putting the stereo speakers to the ground. Homer's record collection, which boasts such gems as Tiny Tim, The Chipmunks, and A Castrato Christmas, leaves much to be desired, but they get some assistance from Barry White (who was conveniently walking by). Bart brings their karaoke machine and speaker outside, turns the volume up to eleven and places the speakers face down on the ground. Then, with White's deep vocals and Lisa's bass guitar playing, they attract hundreds of snakes into the house to the tune of White's 1974 hit song “Can't Get Enough of Your Love, Babe”.
The crowd arrives in pursuit, but they are soon turned around on the subject of Whacking Day by Bart's newfound knowledge. It turns out that the day was actually invented in 1924 as an excuse to beat up the Irish, a fact confirmed by a fellow Irish man. Lisa implores the crowd further, pointing out how the presence of snakes has been beneficial to the community, reuslting in the crowd cheering for the snakes. Mayor Quimby, not knowing that the city has changed its heart, shows up with pre-whacked snakes, but is booed by the crowd. Angered, Quimby calls them "fickle mush-heads"; the crowd admit that he is correct and cheer for him, while the surviving snakes flee away to freedom. Skinner is impressed with Bart's efforts, and welcomes him back to the school to reunite with the rest of the students and upon beginning to say the names of his former acquaintances (Jimbo, Kearney, Dolph and Nelson) realizes in horror that he never unlocked the door and the bullies are still in the basement (the bullies are then shown in the basement, confessing to each other the reasons behind their oppressive behavior and living off a stash of granola bars in the basement). He races back to the school with mountain bikes for the four bullies, hoping that all of them are still alive and that he won't have to face any lawsuits from their families; in case the bullies are dead, he and Willie (who is hauling the mountain bikes with his tractor) will escape to Mexico on said mountain bikes. Willie "agrees" then aptly mutters under his breath, "I'll turn ya in at the first toll booth." The episode finishes with Willie and Skinner driving into the sunset while a mariachi version of the theme song plays in the background.