As the McKenzie family grapples with their mounting financial difficulties, including Jeff's job loss and Pam's insufficient salary to cover their mortgage payments, they find themselves in a precarious situation where they must hide their troubles from their children, Abby McKenzie and Chris McKenzie. Their predicament intensifies when Jeff's well-intentioned yet bothersome brother Hunter, along with his wife Mei and son Ollie, descends upon them for an extended Christmas stay.
In a twist of fate, a chance meeting at an open house brings Jeff McKenzie and Max Mercer together, leading to the surprising discovery of a collection of antique dolls, one with an oddly deformed upside-down face. Carol, Max's mother, shares insights about these dolls' rarity and worth, setting the stage for a series of unexpected events that follow.
As the Mercer family ready themselves to leave for Tokyo, a mishap occurs when Max is mistakenly left behind, thinking he is heading off with the rest of his family. Meanwhile, desperation mounts for the McKenzies as Jeff realizes that one of the rare dolls has vanished and he suspects Max of stealing it. In pursuit of the truth, Jeff embarks on a chase that leads him straight to the chaotic Mercer household, where he inadvertently learns crucial details about their security measures.
Back at their home, Pam becomes aware of Jeff's discoveries and they devise a plan to retrieve the doll under the cover of night. Max, having enjoyed the thrill of being home alone at first, soon experiences feelings of loneliness and boredom. When Jeff and Pam arrive at the Mercer residence, their intentions quickly spiral into a misunderstanding, as Max misinterprets their actions, believing they are planning to kidnap him, which spurs him to call Officer Buzz McCallister.
As the plot thickens and secrets are unveiled, the Christmas spirit seems to take an unexpected turn. When Carol realizes that Max is still at the McKenzie home, she feels an urge to return, reflecting her maternal instincts. On the following day, the McKenzie family learns from their realtor about a potential buyer for their home, which only adds to their stress.
Amid these pressures, Max fortuitously crosses paths with Chris, who unknowingly aids him with a water gun, while the McKenzies remain blissfully unaware of Max's presence. Jeff and Pam, however, are determined to sneak back into their house, only to find themselves in their neighbor's yard, leading to a series of mishaps. Quick-witted Max overhears their struggles and decides to set up traps to protect his space, complicating their efforts further.
As events unfold, the misunderstanding that Jeff and Pam initially harbored gives way to the realization that Max is not merely guarding the valuable doll, but an innocent soda can. The plot eventually thickens to a heartwarming conclusion where the families unite, deciding to let Max stay with them until his mother returns.
Fast forward a year later, as both families partake in a Christmas gathering, they uncover that Ollie was the true culprit behind the missing doll, offering the McKenzies relief. With time, Jeff finds a fulfilling job and extends a generous gesture towards Max by gifting him a soda—symbolizing their unique bond born out of an unforgettable holiday adventure.
