Jeff and Pam McKenzie are trying to sell their house, but have not told their children, Abby and Chris. Jeff has lost his job and Pam's salary is not enough to keep their home. To make matters worse, Jeff's obnoxious and successful brother Hunter, his wife, Mei, and their son Ollie have decided to stay with them for Christmas.

During an open house, Max Mercer and his mother Carol stop by to use the restroom. Max has a brief exchange with Jeff, during which the latter reveals a box of old dolls, including a malformed one with an upside down face. Carol explains to Jeff that dolls with unusual deformities are rare and worth a lot of money. As Max and Carol return home, the whole family is preparing to leave for Tokyo, Japan, for the holidays, with Carol leaving earlier than the rest. Max, annoyed with the fuss, settles in the garage-parked car to watch cartoons and accidentally falls asleep. Unfortunately, Max's family do not realize that their son was sleeping in the car parked in the garage and left him at home, thinking he was in another vehicle with one of the rest of the members.

Concerned about losing their home, Jeff goes to get the doll only to find it is missing. Believing that Max stole it, he locates the Mercer household the next day only to find the whole family hastily leaving. During the commotion, he overhears the security code and sees where the house key is hidden. Telling Pam about it, they agree to get the doll back at night.

Meanwhile, Max discovers the whole family has left and initially has fun, though he quickly grows bored and misses them. Meanwhile, Jeff and Pam arrive at the Mercer's and get inside. Overhearing them talk about getting an "ugly little boy", Max assumes they are talking about kidnapping and selling him. He attempts to scare them away by calling the police. Officer Buzz McCallister (The same Buzz from the first and second films) arrives, but Pam diverts him. Max fears if Officer McCallister realizes he is home alone, his parents may get arrested.

Carol discovers Max was left behind and buys a ticket to head back. The McKenzies go to church the next day and run into their realtor. He tells them there is a buyer, but they must decide by the end of the year, which puts extensive pressure on them. Max arrives and unknowingly converses with Jeff and Pam's son Chris who sympathetically gives him his water gun. The couple spot Max talking with someone and assume she is his grandmother. They resolve to break into the house once again while their family is still at church.

Sneaking around the back of the house, Jeff and Pam end up in the neighbor's backyard instead. Max overhears them once again, with Jeff agreeing to arrive dressed as Santa in an effort to fool him. Max responds by booby-trapping the house to cause difficulty and prevent them from getting close to himself while Jeff and Pam wait for their family to fall asleep on Christmas Eve. The couple fall into Max's traps, during which they discover that Max did not steal the doll, but instead a can of soda. They clear up the misunderstanding, but learn Max is home alone and agree to let him stay with them until his mother returns.

As they explain the whole situation to their family, it turns out that Ollie stole the doll and managed to safely retrieve it, thus ensuring that the McKenzies can s…

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