When the retirement home closes, Marge invites Grampa and two other elderly octogenarians to stay at the Simpsons' house. Nelson becomes Bart's friend, and after Bart defends Nelson for wearing women's underwear, they go to a bully meet. However, when Bart is framed for shooting the leader of the bullies, Bart, Nelson and their group of tormentors get in serious trouble!
Marge and Homer have a make-out session as they arrive home from their date. As the couple pays off the babysitter and ignores the overdue bills and phone calls, Lisa is hollering about Grampa getting kicked out of the elderly home, which finally puts a damper on the couple's romantic night. At the retirement home, Homer and Marge find out that the place is in violation of several codes, for instance, with the soda machine dispensing rats and the mannequin nurses coming out of the home. Because of this, Grampa has to go home with the family until the retirement home is cleaned. However, when Grampa's two buddies from the home, Jasper Beardly and the Old Jewish Man, are left without anyone coming to get them, Marge takes them in (despite Homer's difference of opinion) and the Simpsons allow the pair to stay in the basement of the house.
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 also discloses that he wears his father's hand-me-downs from gorillas. After the class applauds Bart for defending Nelson, Nelson and Bart become close friends. With Grampa and his friends almost dying with every activity, Homer almost dies when he sees the electric bill before Marge revives him. The negligence going around in treating Grampa prompts Lisa to urge Homer to be nicer to Grampa because that will be how his kids treat him when he gets old. Homer, nevertheless, makes a point to say that you do not see him acknowledging the existence of his own grandfather.
Meanwhile, Bart receives a message wrapped around a rock thrown at his bedroom window. When he slides down the drain pipe to meet the culprit, he is kidnapped, even with Chief Wiggum and Lou watching them. When Nelson and the other bullies make their identities understood to Bart, they name him the honorary bully and give him a necklace of retainers. While Bart does not seem too keen on becoming a bully, Homer feels the opposite when Grampa and his buddies induct Homer into the "club" after Homer gets an early breakfast with them. Following a trip to the mall for exercise, Grampa pulls Homer's pants up to make them high-waisted. During a game of bingo, an elderly hits on Homer in front of Grampa, who warns Homer to stay away from those women who just want a chicken dinner out of him.
Later, Homer is in bed with Marge with a chicken meal and turns up the thermostat in the room because he is too cold. Marge also notices that Homer is wearing sock garters. Meanwhile, Bart and the bullies prepare for the Bully Summit held at Krustyland. Chester, the ringleader of all bullies, has called for the meeting. At the amusement park, the bullies are required to turn in their weapons, but Bart gets sidetracked and forgets to turn in his slingshot. When an elderly, more rebellious bully gets a hold of Bart's slingshot during Chester's speech, the shot is on Bart, considering he is the only one in possession of the slingshot.
A manhunt begins to track down Bart, as they all think he did it. So, Bart, Dolph, Nelson, Kearney and Jimbo hide in one of the ferris wheel seats and try to wait it out. The bully that shot Chester gets on the ferris wheel, so Bart and his gang jump off and run. After the S.A.T. Preppers fail to stop them, Bart and the bullies are running for their lives down the street as they are being chased by all the other bully groups in a giant torn up bus. They finally get back to the subway, but the Baseball Furries are there waiting. Bart uses his slingshot to break the street light and distract them, but one stayed behind. Nelson gets talked by a baseball mascot and passes away, after he tells him to "save ourselves."
The boys head back to Springfield just as the sun is rising. However, Bart and the bullies are not the only ones on the beach, as Homer and the other occupants just so happen to be taking a stroll as well. Homer sees Bart in trouble. They have to help them, except the old timers do not really want to and start making justifications. He rushes over and sucker punches the bully, who proclaims he has never been in a fight. He then whines and runs off, and Homer and the gang navigates home.