Nobody ever said being eight years old was easy, especially when you are a middle child in a very large family. For eight year old Kevin McCallister (Macaulay Culkin), he never imagined that screaming "When I grow up and get married, I'm living alone", could possibly lead to his entire family disappearing the next morning. In outright chaos, the McCallister family prepares to spend Christmas with extended family in Paris. After an argument with his older brother Buzz (Devin Ratray), Kevin finds himself published and sent to sleep in the third floor bedroom. Angry, Kevin tells his mother -Kate- (Catherine O'Hara) that "he wishes his family would disappear". Unaware that his wish was about to come true, a power outage during the night causes the alarm clocks to reset and the family to oversleep. In a rush to make their flight, the family miscounts, and no one notices that Kevin has been left behind. Once in the air, Kate realizes that it was not just closing the garage door that they had forgotten, she realizes that they had forgotten Kevin. Frantically trying to get a flight home, Kate tries to get a flight home to her son, only to find that all the flights were sold out, she makes it back to the United States, only to find that she cannot get back to Chicago. Kate ends up accepting a ride with a Gus Polinski (John Candy) and his Polka Band.
Kevin wakes up and is confused, yet thrilled that his wish came true. Running a muck through the house, as many unsupervised eight year old boys would do, Kevin raids Buzz's room, watches 'R' rated movies, eats junk food, and jumps on the beds. His fun comes to a screeching halt as he has a run in with "Old Man" Marley, and recalls the story that Buzz told him about his neighbor who is rumored to have murdered his family and many people on the block many years ago. He also has a run in with Harry (Joe Pesci) and Marv (Daniel Stern), "The Wet Bandits", who were robbing empty homes on the McCallister's block.
Fearful of The Wet Bandits, and not wanting them to realize that he was home alone, Kevin stages the house with music, puppets and more to make it look like a party was going on. Confused, as they were certain the McCallisters were on vacation, The Wet Bandits quickly drove away.
On Christmas Eve, Kevin overhears Harry and Marv's plan to break into his home. Kevin seeks out "Santa Claus" and then attends a local choir performance where he finally gets the courage to speak to "Old Man" Marley. The two speak, and Kevin learns that his neighbor is a nice man, estranged from his family. Kevin, missing his own family, advises Marley to make things right with his own family.
Kevin leaves the choir performance and heads home to protect his home from The Wet Bandits. Setting up numerous booby traps throughout the home, The Wet Bandits break into the McCallister home. Enduring the torture of Kevin's strategically planned booby traps, The Wet Bandits are ready to catch Kevin. Kevin escapes to a neighbors home, calls 911, and believes that he is now safe. Kevin is then caught by Marv and Harry. Just as Kevin has no way to escape, Marley arrives, saving Kevin as he knocks The Wet Bandits out with his snow shovel. The Wet Bandits are arrested for their menacing behavior.
After a night full of adventure, Kevin wakes up on Christmas morning, disappointed to learn that his family has not "reappeared". All of a sudden, Kevin hears his mother calling for him, he runs downstairs into her arms. Moments later, the entire family arrives home. Kevin also notices outside the window that Marley is with his family, Marley and Kevin wave at each other and smile. Kevin and Buzz have a moment of peace, until Buzz enters his room, which has been nearly destroyed by his younger brother, The movie comes to end, as viewers hear Buzz yell "Kevin! What did you do to my room?"