Peter is about to get divorced from Kate and announces that he's living with his new girlfriend, Natalie. He tells his three children, including Kevin, that they are hosting the visit of a royal family and invites everyone to spend Christmas with him and Natalie. After initially refusing, Kevin takes his father up on his offer after being tormented by his older brother Buzz.
Kevin enjoys life at Natalie's mansion, until he goes into the security room, where he runs into Mr. Prescott. While Natalie and Peter are away, Kevin notices his old nemesis Marv and new sidekick Vera (taking the place of Harry) outside the house. Kevin drives them away by flooding the house, but Peter and Natalie arrive while Kevin is cleaning up and Peter doesn't believe Kevin's explanation. He attempts to use the security camera footage to prove himself, but discovers that the camera had been turned off and is caught by Mr. Prescott, who Kevin now believes to be Marv and Vera's ally.
At a party for the royal family, Kevin notices Marv and Vera dressed up as caterers. Kevin has them fall out and break his bedroom window, then has Mr. Prescott frozen in ice. The royal family are unable to attend the party because their flight was cancelled so Peter and Natalie instead announce their engagement. When Kevin breaks his promise not to mess up the special occasion, Peter gets mad and believes that Kevin is trying to end his relationship with Natalie.
Since no one believes him about the heist, Kevin decides to deal with matters himself, and sets booby traps for Marv and Vera and locks Mr. Prescott in the basement. But then he discovers Marv and Vera's real ally is actually Molly the maid, who turns out to be Marv's mother. Kevin gets locked in the basement; after apologizing to Mr. Prescott for misjudging him (and vice-versa), Kevin manages to escape through the dumbwaiter. He then runs to the bedroom and plays a recording of Marv, making it sound like he's insulting Vera, which causes her to break up with him. He also gets Molly trapped in the elevator. Marv and Vera end up swinging from a chandelier, fall and are knocked unconscious. Molly, having escaped, grabs Kevin, but Mr. Prescott knocks her out with a serving tray.
As Peter, Kate, Buzz and Megan arrive, they and Kevin chase Marv and Vera out of the house and trip them up. The royal family and Natalie arrive and Natalie is confused when she sees Marv, Vera and Molly getting arrested. Prescott resigns and the royal family choose to spend their Christmas with the McCallisters instead of Natalie. Peter then breaks up with her and Natalie hyperventilates. The McCallisters embrace as the film ends.