Based on the beloved children's book of the same name, Alexander seems to be the living embodiment of the old adage that "anything that can go wrong will go wrong." He's forever having very bad days, but in his family of six, he's the only one. His family doesn't understand his frustration. So, just before his twelfth birthday, Alexander wishes that the rest of the family could experience his kind of very bad day.
What follows is a comedy of errors as Alexander's parents experience car trouble, work woes, and everything in between. His older siblings each have big dreams they're working toward - his brother wants his driver's license so he can take his girlfriend to the prom and his sister has a huge role in a school play - but both of them find their dreams compromised when they have to deal with fallout of misunderstandings and sickness.
As the family experiences what is undoubtedly their worst collective day, they bond over the hardship.