Marge and Homer are in the throes of full passion as they arrive home from a date, but all romance is out the window when Lisa tells them that the Springfield Retirement Castle has been shut down for massive violations and Grandpa has nowhere to live. Grandpa is picked up by the family, with Homer rudely suggesting they put his dad in a luxury dog kennel before Marge angrily shoots down the idea; while they're there, Marge is moved when she sees that Abe's friends Jasper Beardley and the Old Jewish Man have no one to pick them up, and she says they're coming to stay with the family as well, despite Homer's renewed objections.
At school, Bart and his classmates undress for gym class. Nelson reluctantly changes, but his fellow school students laugh at him when they find out he is wearing women's underwear. The laughter does not cease until Bart points out he's wearing Homer's hand-me-down briefs, which were previously worn by gorillas performing for circuses. After the class applauds Bart for his kind act, Nelson declares he and Bart are now good friends. With Grandpa and his friends needing constant defibrillation around their days of passing gas and reading huge-font online articles, Homer almost dies when he sees the electric bill before Marge revives him. Homer's complaints about the old guys lead Lisa to quietly but seriously point out that she and Bart are getting cues on how they'll take of Homer when he's older, and while Lisa is shocked when Homer reveals that Abe's father is alive but Abe ignores him, Lisa just asks her dad to be nice to Abe, who she calls a "treasure".
Meanwhile, Bart receives a message wrapped around a rock thrown at his bedroom window. When he slides down the drain pipe to meet the rock thrower, he has a sack put over his head and is carried away into the night, even with Chief Wiggum and Lou watching them. Nelson and the other bullies induct Bart into their ranks at an abandoned country club. Back home, Homer's Lisa-inspired efforts to be nice and helpful to Abe and his friends leads to Homer surprisingly enjoying the senior citizen lifestyle of early breakfasts, mall-walking and Bingo, and they induct homer Homer into their "club". Unfortunately, Homer's appreciation for all things elderly has a negative effect: Marge finds his new behaviors irritating and sadly tells Patty & Selma that while she didn't mind when he got bald and then fat and then fatter, she never expected or wanted him to act older than her.
Meanwhile, Bart and the bullies prepare for the Bully Summit held at Krustyland. Chester, the leader of all bullies, has called for the meeting. All bullies are required to turn in their weapons, but Bart gets sidetracked and forgets to turn in his slingshot. When an elder, more rebellious bully uses Bart's slingshot during Chester's speech, the guilty bully blames Bart and he's left with Nelson, Dolph and Jimbo to try to get back to Springfield without getting beaten or killed by all the enraged gang members.
A hunt, based on The Warriors, begins to track down Bart as the perpetrator. Bart, Dolph, Nelson, Kearney Zzyzwicz, and Jimbo Jones hide in the Ferris Wheel and try to wait it out. The Bully that shot Chester is on the wheel, too so they jump off and run. After the S.A.T. Preppers fail to stop them, Bart and the Bullies are running for their lives now down the street, chased by all the other bully groups in a giant torn up bus. They finally make it back to the subway, but the Baseball Furries are there waiting. Bart uses his slingshot to break the street light and distract them. One stays behind, however and Nelson sacrifices himself for the others. The boys make it back to Springfield just as the sun is rising. Bart and the Bullies aren’t the only ones on the beach though. Homer and the old guys just so happen to be taking a stroll too. Homer sees his son is in trouble and wants his new/old friends to help, but they demur in cowardly fashion, so Homer goes off at his not-that-fast top speed and punches the enemy Bully before he can hurt Bart. In a flash, the Bully begins crying and says he's never actually been in a fight, and he runs off with his allies while Bart, Homer, Nelson and his crew, and Abe and HIS crew walk slowly along the beach back towards Springfield.