"The Kids Are All Fight" is the 19th episode of season 26. On November 11, 2014, Al Jean wrote on twitter that this would be a Bart and Lisa flashback episode. It takes place when Lisa was two years old and Bart was four years old.
When Homer gets an old film roll developed, the family takes a trip down memory lane to see the origins of how Bart and Lisa first started getting along with each other.
Homer goes to Moe's Tavern with a new suit, but when he tries to pay for the beer, he finds out that his pocket is full of old stuff. One of the items is an old film roll, Moe tells Homer that his bar is legally a black room and Homer plans to develop it. After bringing the family to see the photos, they realize why the film was never developed because it was full of photos of young Bart and Lisa fighting. The kids get curious about how they stop fighting each other all the time, and Marge tells them a story, that leads to a flashback.
The flashback takes place six years ago when Bart was four years old and Lisa was two. They went to a library to hear a storyteller, but the siblings start hitting each other with books, and they are expelled from the library. Later that night, Bart is scared because of his clown bed, and he does a drawing about the bed, but, while he is drawing, Lisa takes his pencil claiming he stole it, and shows him that she could write his name better than Bart himself. Bart gets angry and starts hitting her with a toy, but Homer sees Bart hitting Lisa and gets angry and shouts "Why you little!" and strangles Bart, but Bart manages to grab a lampshade and hits Homer in the head with the lampshade, Homer asks where is Bart learning this violence, making Marge to get upset
Marge was so frustrated about they kids that she start having nightmares, and they choose to take them to a counselor and the couple is told them that one of their kids is smart and good (Lisa) and the other is dim and evil (Bart). Back at the house, Ned proclaims to help them, letting the kids with Grandma Flanders for them to enjoy a day without the kids, but they enjoy their day at the house instead of going out with the Flanders. At the Flanders house, Grandma Flanders appears to die, leaving the kids unsupervised. Bart and Lisa try to go back home, but the door is locked. They hear an ice cream truck, Bart takes his tricycle and Lisa takes her pedal car to buy ice cream, but they get lost in the city.
Later, Ned finds his Grandma lifeless and the Simpsons kids gone, much to Homer and Marge despair. In the city, Bart goes into an alley, where he finds younger Kearney, Dolph, and Jimbo. The bullies take his tricycle, but Lisa starts crying and they choose not to pick on them. At this moment, Bart realizes Lisa was smart and they make a good team. Then, they go to the Retirement Castle, where Grandpa Simpson takes care of them. Later, they meet Milhouse and tell him they're lost. Milhouse goes to talk about them to his dad, but he gets distracted by cartoons. They had to go because of a "big dog" (a Chihuahua), and they go down a hill, but Lisa couldn't control her pedal car, so Bart saves her moments before the car hit a tree and exploded.
Homer and Marge try to find the kids desperately, and they eventually find them, on top of the Tire Fire. Homer tries to bend up a tree to save them, but the branch he was holding broke, catapulting the kids back to the Simpsons House into the clown bed, that breaks apart. Bart hears another ice cream truck and goes to get find it but Lisa holds him back. Back to the present time, Marge tells that, since that day, they got along and they were able to have Maggie. In the ending of the episode, Dr. Hibbert tells Ned that his Grandmother survived, and Moe calls Homer "Father Of The Year".