A deadly virus has swept the world, killing off everyone over the age of twelve in the span of a month or so. In the town of Glen Ellyn, Illinois, outside of Chicago, ten-year-old Lisa Nelson and her younger brother Todd are surviving, like all the children in the story, by looting abandoned houses and shops. Although there are abandoned cars in every driveway and lining every street, Lisa is the first child to think of driving one. She is also the first to think of raiding a farm, and the first to look at the dwindling supplies in stores and deduce that groceries come from warehouses.
She finds a supermarket warehouse and raids it, enlisting the help of Craig Bergman, a neighbor boy two years older than her, but makes clear to him and all the other children in her neighborhood that the entire warehouse and all its contents are her exclusive property, not to be shared unless she chooses: she assures them all that she'll burn the warehouse and everything in it rather than be forced to share against her will.
She considers relocating to the farm, but decides against it because it is difficult to defend (other children are starting to form gangs) and because "planning and getting the world back to the way it was, with schools, and hospitals, and electricity" are much more "exciting" than "hiding away on a farm ... digging in the dirt all day".
Lisa and her friends are approached by the "Chidester Gang", led by Tom Logan. Suspecting that Lisa has a source of supplies, Logan offers a food-for-protection deal, which Lisa declines. Unhesitatingly taking charge, she forms her block-long stretch of Grand Avenue into a militia, armed with guns, Molotov cocktails, and primitive weapons. When the militia proves unsuccessful at defending the "Land of Grandville" against "the fearful and cruel army of Chidester and Elm", and Lisa's house is lost, Lisa comes up with the idea of moving the "child-families"—and the entire contents of the warehouse—into the local high school, and transforming it into a fortress-city. Within the city Lisa is the only authority, by virtue of the fact that she saw the abandoned high school and thought of moving there: this has earned her sole title to the "City of Glenbard" and everything in it.
After completion, and a year, things proceed according to plan until Tom Logan and his gang manage to stage a successful attack on Glenbard, during which Lisa is shot in the arm. Todd and Lisa's friend Jill rescue her, and Jill performs basic surgery to remove the bullet from her arm, dosing her with whiskey for pain relief. When Lisa recovers they retake the city of Glenbard from Tom, who has meanwhile learned that conqueror and leader are two very different things. Glenbard's "citizens" have shown no sign of rebellion, or of preferring Lisa's leadership to Tom's (or vice versa), but Lisa lectures Tom into relinquishing control of the city to her.
The book ends with a foreshadowing that the citizens of Glenbard will at some time be forced to face far larger armies, led by now extremely powerful dictators, tyrants and warlords. If any semblance of a free society is to exist in the new world, the citizens of Glenbard must make themselves capable of protecting and growing it by gaining in knowledge, power, and organization, and at the same time continuing to incorporate leadership and respect for the individual person into their society.