Very spoilery, though most of it you’ve probably seen before… (My 2/5 review is here.)
Pam and Jeff Mackenzie are selling their family home at an open house viewing day. Their children Abby and Chris don’t know they are moving, but since Jeff lost his IT job they can’t survive in the house on Pam’s teacher’s salary alone.
Max Mercer and his mum Carol are driving through snowy Chicago. Max needs the loo, so they stop off at the open house viewing to use the bathroom. While there, Jeff shows them a box of his mother’s old dolls, including an ugly boy doll with its head on upside down.
Carol recognises it as a Schneider Kinder doll and says they can be valuable, and Jeff makes a joke about not getting that much when he sold his old kids. Overheard by Jeff, Mum and Max argue about going home, and she threatens to leave him behind with his granny when they fly to Tokyo the next day.
Back at the Mercers’ it’s pre-holiday chaos. Outside, their house has a sign saying it’s protected by… McCallister Security. Carol is on the phone, arguing with the airline as their flight has been rebooked across two planes. Max goes to the car to watch cartoons to escape the noise.
At the Mackenzies late that night, Jeff’s brother Hunter and sister-in-law Mei arrive for Christmas with their 4 year old son Ollie. Jeff googles the Schneider Kinder upside down doll, discovering it’s incredibly rare — the last one sold for $200k. He can’t find his doll though, and thinks Max has taken it. He knows Max and Carol’s full names as they used them with each other in a spat at the open viewing, so early the next morning Jeff drives there.
Carol has already left for Tokyo, and the uncles and cousins trying to catch the next flight assume Max has gone with her. Everyone leaves and Jeff peers in, seeing what he thinks in he shape of the doll in Max’s coat. He unlocks the door, having seen one of the uncles leave the key under a flowerpot, but is disturbed and leaves.
Max wakes up and realises he has the house to himself. He entertains himself eating sweets, riding his scooter on the treadmill and sliding down the stars on an ironing board (I know right? Who irons anymore?)
Pam is upset thinking about losing their house, so she and Jeff decide to pop out and let themselves into the Mercers as they have the key. They go in with the wrong alarm code though, which summons the police via the alarm company. Max overhears Pam tell Jeff they only need to find the ugly little boy and an old lady will pay $200k for him, and Max thinks they’re talking about him. He tells their HomeBot to call the police, and when it replies in German Pam and Jeff assume it’s someone else in the house. A police car pulls up; the cop is Buzz McCallister. They fob him off pretending they live there, much to Max’s horror as he watches out of the window — but he decides it’s too risky calling the police again and telling them he’s home alone in case they put his parents in prison.
In Tokyo, the two halves of the family are reunited and discover Max was left behind. Carol rushes to the airport and manages to get a standby ticket back to Chicago. Her sister-in-law calls the police about a boy left home alone but Buzz thinks it’s a prank call from his brother Kevin, which he makes every Christmas.
At a Christmas Eve service with a toy drive for underprivileged children, Max meets Chris, explaining to him and the lady running the toy drive that he’s just lost his parents. She lets Max take the massive Nerf-type gun Chris has brought for the toy collection. Jeff and Pam sneak out of the service to go to Max’s house again but they break into next door by mistake and fall into their icy swimming pool. They decide to return at midnight with Jeff dressed as Santa, but Max overhears them talking about an ugly little boy in his sack and again thinks they’re coming for him. He starts setting up boobytraps.
Jeff and Pam wait until everyone is asleep and, with Jeff dressed as Santa, return to the Mercer house. Max has baked cookies laced with hot sauce, buttered the staircase, created a fire-starting doormat, frozen over the drive and set up a flour bomb, among other tricks. The Mackenzies get a bashing, with Pam becoming angrier and angrier at Max.
Eventually all three manage to talk, explaining the misunderstanding. Max says he didn’t steal the doll, and admits he’s home alone. Pam says if they’d known they wouldn’t have acted like this. As a final indignity, a heavy light falls on all three.
Pam and Jeff take him home with them, and have to explain to their own children that they’re selling the house. Pam says that what makes the house is the family in it, and Max agrees that without his parents with him, his house didn’t feel like home.
Ollie appears at the top of the stairs holding the ugly boy doll. He throws it, and everyone leaps across the room — Max catches it, throwing himself on the floor. His mum arrives from the airport and they’re reunited. They go home and she opens the door to the destroyed house.
One year later, Carol, her silent husband and Max are having Christmas dinner at the Mackenzies, who were able to keep their house, holiday in Europe and fix up the Mercers’ house with the proceeds from the ugly boy doll. Jeff also has a job. They toast new friends, family, and Max for bringing them together.