In a surprisingly plausible setup, Kevin is mistakenly accounted for, and the family heads off without him. It's interesting that, in a subtle way, his parents never noticed him missing because of their deliberate desire to separate themselves from their children. First they delegate the headcount to an older sibling, and second, they put all the kids in coach while they fly first class. They're two-thirds of the way across the Atlantic before they realize they are missing a child. Meanwhile, Kevin believes that somehow his wish has come true.
Life is a dream come true.
He can jump on the beds, watch R-rated movies, eat gobs of ice cream and potato chips, and dig through his big brother's secret box. Slowly, the novelty wears off and he settles into the mundane. He also starts to miss his family, not just for their company, but for the security they bring. The furnace in the basement is a scary monster, the old man next door is a snow-shovel murderer, and two crooks are trying to break into his house. Macaulay Culkin is cute but he isn't much of an actor.