The film follows the exploits of Alexander Cooper (Ed Oxenbould), an ordinary 11-year old boy, and his "terrible, horrible, no good, very bad day." Alexander is left out by his family; his older brother, Anthony (Dylan Minnette), his sister, Emily (Kerris Dorsey), his mother, Kelly (Jennifer Garner), his dad, Ben (Steve Carell), and their infant baby son, Trevor (Elise/Zoey Vargas).
The day before Alexander's twelfth birthday, he wakes up and finds gum in his hair, clipping it off with a pair of scissors. Anthony is trying to earn his driver's license so he can drive his girlfriend Celia (Bella Thorne) to prom, Emily is rehearsing for the title role in her school play of Peter Pan, Kelly is working for a publication company that is publishing a new children's book, and Ben, who has been unemployed for several months, has landed a job interview as a game designer for a video game company.
That same morning, Alexander attends school where he experiences another series of misfortunes, as well as finding out that his friends, including his crush Becky Gibson (Sidney Fullmer), and friend Paul (Mekai Curtis), will all be attending Phillip Parker's birthday party instead of his due to his expensive items and popularity. Alexander tries to tell his family how his day has gone, but finds out that no one really even listens to what he says. That evening, Anthony upsets Celia during a phone call while yelling at Alexander saying he is a "idiot brother", and Emily rehearses her lines in Kelly's Volvo and leaves a light on to rehearse for her play. Alexander serves himself a makeshift birthday sundae and wishes for his family to experience what he does everyday.
The next morning, Alexander wakes up to find his family in disarray; his parents have overslept, Emily is sick with a cold, and Anthony has found out that he and Celia have broken up. The battery in Kelly's car is dead, because Emily left the light on all evening while rehearsing, therefore Ben has to take Trevor with him to the interview, after dropping Kelly off at work. At school, Alexander is told that the other party for Phillip Parker has been cancelled due to Chicken Pox, and calls Ben and asks him to plan a party for him. Kelly is informed of a misprint in the book they are publicizing, and needs to stop Dick Van Dyke from reading the book at public reading later. Ben goes to the interview with Trevor and meets Greg (Donald Glover), although the meeting goes awry when Trevor ingests a highlighter.
Kelly manages to arrive at Dick Van Dyke's reading, though she is unable to convince him of the book's misprint. Van Dyke reads the book with the inappropriate material, humiliating himself and the publishing company. Meanwhile, Ben has purchased Cough syrup for Emily's worsening symptoms and takes Anthony to the Department of Motor Vehicles, where he fails his driving test and damages the family van. Alexander tells the family of his wish and apologizes, but Ben says "Apology not accepted" because they can still have some good in it. Afterwards, they all go to Emily's play, which is inadvertently sabotaged by her impaired actions due to her overdose of the medicine. The game design firm call Ben and ask him to meet them at a Japanese hibachi restaurant for a second meeting. The family, joined by Celia, go the restaurant where Ben has a minor accident, which causes embarrassment to the employers. The whole family consoles Ben, admitting that they will overcome whatever else the day has in store for them, Alexander states "You just gotta have the bad days so you can love the good days even more", along with Anthony deciding not to go to prom with Celia, stating that his family is more important and also breaks up with Celia completely.
The family returns home, and discover that Ben had hired a rental animal service to recreate an Australian petting zoo for Alexander's birthday party. The family then decide to pitch in to try and salvage the day by helping host Alexander's party. Ben and Kelly receive calls; Ben got the game design job, and Kelly has been informed that the celebrity reading went viral and has created publicity for the book. Reunited, Ben then brings out the cake, with Alexander wishing for more days like the one they shared together.
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