The Cooper family finds themselves in a less-than-ideal situation as they drive home in their dilapidated minivan. Alexander (Ed Oxenbould) laments that their day has taken a turn for the worse. His baby brother Trevor (Elise Vargas, Zoey Vargas) is covered in green marker, while his older brother Anthony (Dylan Minnette) is clad in a peculiar blue tuxedo. Their parents, Ben (Steve Carell) and Kelly (Jennifer Garner), endeavor to maintain a positive attitude amidst the chaos. Alexander holds himself accountable for their misadventures, citing that the day's troubles largely stem from him. The situation escalates when the family arrives at home and is greeted by a shocking surprise, leading them to scream and slam the door shut.
To backtrack a day, Alexander receives an invitation to the birthday party of Philip Parker (Lincoln Melcher), the most popular kid at his junior high—an unfortunate coincidence, as it falls on the same day as his own birthday. The morning begins on a sour note when Alexander discovers gum stuck in his hair and resorts to cutting it out. Due to his parents' busyness, he has to carpool with his crush, Becky Gibson (Sidney Fullmer) and her family. However, disaster strikes when he trips and lands face-first in front of Becky. To top it off, her brother Elliot (Reese Hartwig) humiliates Alexander by mass-texting the entire school with photoshopped pictures of him.
Meanwhile, Kelly's professional life takes a hit when her boss, Nina (Megan Mullally), tells her that a new children's book about potty training is on the verge of her promotion to Vice President—if all goes well. Anthony, eager to impress his girlfriend Celia (Bella Thorne), struggles to keep their relationship afloat amidst ambivalence about prom plans.
At school, Alexander attempts to navigate his awkward situations, from setting fire to Becky’s notebook during science class to dealing with the class gerbil that he is tasked with taking home. Back at the Cooper household, Alexander's day continues to spiral when he accidentally destroys Trevor's favorite bumblebee pacifier while attempting to wash it, sparking more tears from Trevor.
During a family dinner, the Coopers share their successes, only for Alexander to recount his unfortunate escapades, only to be overshadowed by Trevor’s cries. Seeking solace, Alexander confides in Anthony about feeling misunderstood, but it quickly devolves into a misunderstanding that leaves him feeling even more isolated.
The next day brings little relief as the chaos continues with a flat car battery, a printing error on Kelly’s book, and Anthony’s disastrous driving test that leads to conflict within the family. Misfortunate events seem to plague the Coopers, including Anthony being suspended from school.
As fate would have it, the tides begin to turn when Alexander learns that Philip's party has been canceled due to chicken pox, potentially allowing all his classmates to attend his birthday. With hope rekindled, Alexander invites Becky again, and she gives a vague "maybe." Amidst this, they brave further family misadventures, including a disastrous dress rehearsal for Emily’s school play and a comically embarrassing dinner at a hibachi restaurant.
Although the day appears chaotic, it culminates in a surprise birthday bash for Alexander featuring an unexpected alligator—an Australian petting zoo treat gone awry. To top it off, Alexander’s parents receive good news on their professional fronts as well. As the day wraps up, amidst laughter and fond memories, Alexander makes a heartwarming birthday wish for more days just like this one, prompting a united, albeit exasperated, "NO!!!!!" from his family. He concludes that despite the setbacks, this day, riddled with misfortunes, turned out to be the best day ever.
