The Cooper family is driving home in their busted minivan. Alexander (Ed Oxenbould) says that they are not having a good day. His baby brother Trevor (Elise and Zoey Vargas) has his face covered in green marker. His older brother Anthony (Dylan Minnette) is wearing an odd blue tuxedo. His parents, Ben and Kelly (Steve Carell and Jennifer Garner), try to stay positive. Alexander claims this is mostly his fault. As soon as the family gets to their house, his sister Emily (Kerris Dorsey) gets out of the car and pukes. They open the door of the house and see something that frightens them, making them scream and slam the door.
One day earlier.
Alexander wakes up to an invitation to a birthday party for Philip Parker (Lincoln Melcher), the most popular kid at his junior high school. This party also happens to be the next day, which is when Alexander is having his birthday. When he gets to the bathroom and finds gum stuck in his hair. He cuts the chunk of it out. Since his parents are super busy, Alexander has to carpool with his crush, Becky Gibson (Sidney Fullmer) and her family. He walks to the car and trips on the sprinklers, landing face-first on the grass in front of Becky. On the ride to school, Becky's brother Elliot (Reese Hartwig) takes a picture of Alexander and uses an app to put his face on the bodies of women in bikinis.
Kelly works at a publishing company where they are set to release a new children's book on potty training. Her boss Nina (Megan Mullally) tells Kelly if this book takes off, Kelly will be promoted to Vice President.
Anthony gets to school to meet with his girlfriend Celia (Bella Thorne), whom he says is the hottest girl in school. Celia grills Anthony for not sounding flirty in his texts to her (ie, ending them with "xoxo"). She wants to go to prom in a limo, but Anthony offers to drive her after he passes his driver's test. Celia isn't thrilled with that idea.
Alexander walks to class with his best friend Paul (Mekai Curtis). They see that Elliot mass-texted everyone in school all the photo-shopped pictures of Alexander, and everybody laughs at him once they see him. Philip comments, "Nice boobs." Paul also mentions that he's going to Philip's birthday party, despite the fact that Philip's birthday is a week later. In class, the teacher is ready to assign the students an assignment on different places around the world, and Alexander requests Australia due to his fascination with the place. The teacher grabs a globe and spins it to pick what the students will learn, and he makes Alexander do research on Djibouti while Philip gets Australia. Later, Alexander has science class and partners up with Becky. He tries inviting her to his birthday party, but she is already going to Philip's. When he's not paying attention, he accidentally lights Becky's notebook on fire with the Bunsen burner, and then sets the periodic table of elements chart on fire. The teacher puts the fire out with an extinguisher, and the whole class sells Alexander out. As he ends the day, another teacher asks him to take home the class gerbil.
At home, Trevor drops his favorite bumblebee pacifier. Ben asks Alexander to wash it, but he ends up dropping it in the garbage disposal. He goes to grab something to pick it out, but Ben turns it on and destroys the pacifier, leaving Trevor to stay crying.
During dinner, the family lists off their good news. Kelly mentions her possible promotion. Ben scored an interview with a video game designing company. Emily is going to be the lead in her school's production of Peter Pan. Anthony is excited to take Celia to prom and to pass his driver's test. Alexander mentions all the awful things that happened to him that day, but Trevor starts crying, so his parents divert their attention to him, while Emily retreats to the car to rehearse.
Before going to bed, Alexander tells Anthony that he thinks nobody understands him. It sounds like Anthony is sympathizing with him and showing him love, until he makes a comment on "that cute little butt", and we see he is talking on the phone with Celia. Anthony berates Alexander out loud, making Celia think he's talking to her, and she angrily hangs up. At midnight, Alexander goes the the kitchen and makes himself a birthday sundae with a candle in it. He talks to the gerbil and says that he wishes his family could understand how he feels and what it's like to have "a terrible, horrible, no good, very bad day". He blows out the candle.
The next morning, Alexander wakes up before his family. Ben and Kelly spring out of bed, as Kelly realizes she is running late. Emily has tissues all over her bed, having gotten a cold overnight. Anthony discovers a huge zit on his forehead. Kelly rushes everybody out of the house since her car is dead after Emily left the light on and killed the battery.
After getting the kids to school, Kelly runs into work to learn that there was a misprint in the book that replaced the word "jump" with "dump" on not just the cover, but every page inside. To make things worse, she remembers that there is a celebrity reading at the nearby book store. She borrows a co-worker's bike and gets there with helmet hair. The celebrity is Dick Van Dyke. Kelly misses the chance to warn Dick about the mistake, and he ends up reading the book that makes it sound like the book is all about taking a DUMP.
Anthony catches up with Celia, who is still mad at him and breaks up with him. He later gets her out of class to beg her forgiveness and for her to still go to prom with him. She accepts. Anthony gets so excited that he jumps up and hits a banner, which pulls down two trophy cases, breaking both of them. He gets suspended.
Ben has to take Trevor with him to the job interview. He meets the interviewers Greg (Donald Glover), Logan (Joel Johnstone), and Joaquin (Ben Greene). Although he is significantly older than these people, the three of them like what they saw with Ben's past work experience. Trevor keeps bumping around the walls, so Ben brings him closer. Trevor grabs a green marker and rubs it all over his face and mouth.
Alexander, meanwhile, is having a significantly better day. Paul tells him that Philip's party is canceled because Philip came down with the chicken pox, so now everybody can go to Alexander's birthday. Also, Elliot got in trouble for the mass texts. Alexander finds Becky and invites her again to his party. She gives him a "maybe".
Ben picks his kids up and goes to the pharmacy to get medicine for Emily, who is congested but still needs to be back at school for the dress rehearsal for the show. Kelly meets up with them after escaping the angry parents and Dick Van Dyke.
The family takes Anthony to his driver's test. He gets stuck with Miss Suggs (Jennifer Coolidge). On the road, she critiques him for driving ten miles below the speed limit. He gets a phone call from Celia, but knows he shouldn't answer. Suggs implies that he should take the call since Celia probably wants to talk prom. Anthony answers the phone, and Suggs yells at him to put the phone away. This distracts him, and Anthony almost collides into a car. Panicked, he swerves into multiple parking meters, severely damaging the car. Suggs fails him, and as Anthony opens the door, a car drives by and breaks it off. This brings everybody to argue about their problems, with Anthony and Emily blaming Alexander for what happened with them, and their parents arguing back, until Alexander tells them to stop and admits that he made the wish for them to see what it's like to have a rotten day like he usually does. Ben tells them not to let this keep them down, and they head off to their next destination.
The first stop is to a tuxedo shop for Anthony, but the tuxedo he ordered was mistakenly given to someone else. He is given the blue tuxedo. The Coopers then get to school for Emily's show, but she is "drunk" off too much cough syrup. The show goes on, and Emily makes her entrance as Peter Pan too early. She giggles and flies around the stage with her harness, clumsily bumping into the set pieces until the kid playing Captain Hook brings her down. She gets on her feet and throws glitter into the audience. Backstage, Ben tries to comfort Emily after that debacle. She pukes on him. He then gets a phone call to meet up with Greg and his friends at a hibachi restaurant. Alexander gives him a pirate's blouse to change into.
On the way to the hibachi restaurant, the Coopers pick Celia up for prom. She is none too pleased to be riding in a wrecked minivan that smells like vomit, or that they're going to a hibachi restaurant. When they get there, Anthony and Celia sit apart from the family. Their school friends come over, also ready for prom. One of them asks Celia if they love the place, and she lies that she does. Ben finds Greg and his friends at their table, and they are amused by him wearing the pirate's blouse. He sits with them and has sake bombs. The chef also flings shrimp into Ben's mouth, to the amusement of the other patrons. Then Ben's sleeves catch fire, forcing him to stick his arms in the fish tank. Embarrassed, he walks outside and kicks some garbage cans. The family rejoins him, and Alexander tells him that even though he is trying to be positive, everybody is gonna have bad days and that they must appreciate the good days. Anthony then ditches Celia to be with his family after she makes a rude comment on his family being crazy. He then gives Alexander a birthday punch.
The Coopers make it home to find an alligator in their house. This is part of an Australian petting zoo that Ben and Kelly planned for Alexander's birthday. He is thrilled, but the people setting up the pens say that a kangaroo got out. They manage to get the alligator in the pool, however. The kangaroo passes by, and Ben runs after it. He finds it in a neighbor's yard, and the kangaroo kicks him. They manage to bring it back, though.
All of Alexander's friends turn up for his birthday, including Becky. She says the party is pretty awesome. Ben and Kelly both get phone calls. Ben gets the job and Kelly is told that Dick Van Dyke's reading of the book went viral, and that the book is already selling fast. They look outside and see that their kids are enjoying themselves. They bring out a cake for Alexander. He makes a wish that there be more days like this one (which the rest of the family say "NO!!!!!"). Alexander closes the film by saying that for a pretty bad day, this one turned out to be the best day ever.