The episode opens with a dodge ball match between the 4th and 5th graders. Bart ends up hitting the last 5th grader, but he catches it after it comes back down, making the 5th graders win the match. In the days that follow, the 4th and 5th graders commit acts of war against each other, even dragging the staff into it when a 5th grade teacher insults Edna Krabappel's class and it sparks a mass teacher brawl in the staff room. Eventually it gets to the point that they organize a fight after school. Before the fight begins, however, Bart realizes that one of the 5th graders has the same scar as he does, in the shape of a sword on his fist.
Bart confronts Marge about the scar. Marge explains that when he was in preschool, he was in a "Mommy and Me" class with three other kids. Marge became very close with the other mothers, and they became "the Cool Moms". However, the other kids were a bad influence on Bart. But she does not explain the scar. Marge then decides to get back together with her old group.
Marge's old friends reconcile while Bart gets together with the other kids, where he realizes he still does not know where the scar came from. Marge and her friends decide to get together every Tuesday. Meeting with the other kids every week, their antics become more and more dangerous, so Bart decides to break up the group. Knowing they broke up before, Bart realizes it must have something to do with the scar and consults Dr. Hibbert, who suggests he ask Comic Book Guy.
Comic Book Guy is reluctant to explain but, after some coercion, he reveals what happened: About seven years ago, on the 4th of July, he was in charge of the fireworks. While Marge and the other moms talked, Bart and the boys stumbled off towards the fireworks controls and pressed all the buttons, setting off all the fireworks. In the fiery explosion that followed, Comic Book Guy's sandwich was hit by a firework, sending the burning hot sword-shaped skewers onto the boys' fists, branding them.
Knowing it was an explosion that split the group apart, Bart and Milhouse create a big ball of firecrackers to break them up again. When Marge catches them in the act, Bart confesses. That evening, Marge tells the group of Bart's scheme to break them up, and they tell her Bart was always a bad influence to their kids. Marge becomes angry and leaves the group in a rage. After Marge's outrage and leaving of the group, the remaining women all start making out with each other.