The film begins with a busy home of Peter and Kate, and their children and extended family, as they prepare for their trip to Europe. What we see is the reality of life in suburban middle class America, parents stressed about getting their kids to do what they say, Kevin being picked on my his siblings who don't show him kindness and the reality that pretty much everyone in his family shuffles him off or ignores him. So, when the family forgets Kevin when they leave for Paris he wakes up and is excited to be home alone. Kevin indulges in the things his parents won't let him do normally, eating huge bowls of ice cream, watching violent movies and shooting his bb-gun in the house.
Soon after, Marv and Harry, a couple of local thieves, have set their eyes on robbing Kevin's home as they think the family is gone. Kevin learns of their plans and thwarts it by making it appear the house is full of people by using cardboard cutouts to trick them. But Harry and Marv soon learn the house is occupied only by Kevin and they plan to come Christmas Eve to rob it. In the meantime, Kate has figured out that they left Kevin and she's making her way back home by any means possible. Kevin misses his family after having been away from them even though they are hard on him. He is able to stop the burglars by setting traps for them and eventually the police arrest them.
The film emotional note is that regardless of how hard our family is on us we always wish to be with the people we love rather than be alone, and that a mother will do anything to get back to her son. The film ends with Kevin's brother Buzz screaming at Kevin about his room being torn apart. And in this moment all of the sentimental reconciliation and love is thrown out the window as Buzz is ready to kick Kevin's butt...and we're back to life as usual in a home full of kids.
