Tired of his parents making rules and "ruining" things for him, Samuel "Sammy" Benson (Billy Unger) wishes upon a star that kids should rule the world. Meanwhile, Chaz is upset over his scientist father using him as a guinea pig for his latest experiment, the Demystifier, and messes with the machine which causes the entire town to be engulfed in a thick white mist.
When Sammy, his sister Carla (Ariel Winter) and their grandparents return from their family vacation, they discovered all the adults in town (Including their parents) are acting like young, immature kids while the children take on important jobs. Initially, Sammy and Carla were confused yet happy with the change until they really started missing the old ways but Chaz thinks otherwise and believes the entire world is better off being demystified. It's now up to the siblings to fix their topsy-turvey town before Chaz makes it "Opposite Day" permanently.
This should not be confused with the Main Page of the same name.
This film contains the following tropes:
- Be Careful What You Wish For: Sammy wished that kids ruled the world while at the same time the Demystifier engulfs the entire town in a mist that switches the identities of adults and children. With nobody his age wanting to play with him anymore because they are too busy with their jobs on top of having to take care of his parents, Sammy regrets taking his life for granted and wants things to go back the way it was.
- Drowning My Sorrows: A kid-friendly version where Sam goes to a malt shop and spends the entire afternoon drinking milkshakes after realising that being an adult was not all cut out to be and wants things to go back to the way it was.
- Hide Your Children: Teenagers seemingly don't exist in the film as they don't have any age-group counterparts. Tweens like Chaz however do appear and take the place of adults in the demystified world.
- Instant Awesome Just Add Ninjas: Chaz hires ninjas at some karate school as bodyguards so no one can interrupt him demystifying the whole world.
- Opposite Day: In the title and its premise where kids take on the role of the adults and vice-versa. Additionally, the elderly are regarded as babies and the reverse applies.
- Plot Hole: Grandparents are the equivalent of babies for the demystified town but can be still trialed, convicted and jailed like actual adults.
- Shopping Cart of Homelessness: There is a notable homeless old man seen pushing a trolley full of cans in the opening montage. His role is taken by a little kid after the demystification.
- Shout-Out: When Grandpa Jack and Grandma Martha had to stand in a police lineup, the five other kids taking part are dressed up as The Usual Suspects.
- Something We Forgot: Sammy and Carla finally go on a family vacation with their parents when they suddenly realize that their grandparents are still stuck in jail since the beginning of the movie. Cut to Grandpa Jack in his cell with Grandma Martha, showing off his new prison tattoo reading "The End".
