By the time he got to school, not only is he humiliated, but he learns that his best friend Paul is going to Phillip’s party. He tries to ask his school crush Becky Gibson, but she refuses as well. Afterwards, Alexander accidentally burns down the periodic table with her notebook and gets blamed for it. While waiting for a long time outside of school, Alexander is put in charge of Melvin, the class guinea pig.
Later that night, Alexander tries to talk to Anthony about his bad luck. While Alexander thinks that Anthony loved him as a brother, Anthony is actually talking to Celia on the phone. While Anthony is talking to Alexander, Celia, thinking that Anthony is trash-talking to her, then abruptly hangs up, and Alexander hears Anthony calling him an “idiot brother.”
At midnight, Alexander makes himself a birthday sundae and tells Melvin he wishes that his family would see what it was like to have a “terrible, horrible, no good, very bad day.”
The next day, Alexander is the first one to wake up. While heading down the hallway, he sees Emily having a cold, and then wakes up his parents, who were oversleeping. As Alexander leaves the bathroom, Anthony comes in, and reacts in horror when he looks in the mirror and sees a zit on his forehead.
At school, Alexander hears from Paul that Phillip’s birthday party is cancelled, as Phillip has caught the chicken pox (which is shown in a picture that Paul got in a text from Phillip, which he shows Alexander on his phone). This implies that Alexander’s party was back on. (As Paul puts it: “His pox is your gain.”) Becky even mentions showing up. Alexander even hears that Becky’s brother Elliot Gibson, the boy who made the mean photoshops about him, had gotten suspended.
Throughout the day, while attempting to convince his family that their bad luck was his fault, Alexander learns that Trevor got green ink all over his face. Alexander and his family later see Anthony accidentally trashing their van, which resulted in failing his driving test. Emily gets drunk because of drinking too much cough medicine, and ruins the Peter Pan play as a result.
By the time the Coopers picked Celia up for the prom, Alexander and most of his family play music by using their fists and their mouths. At a Japanese restaurant, he sees his father catching food into his mouth eight times in a row, and then accidentally set his sleeves on fire (which he put out using water from a lobster tank). While Ben walked out, feeling disappointed (and trying to hide it), Alexander follows, along with the rest of the family. After realizing that they got to spend the worst day together, they begin kicking trash cans around.
After going home, they discover a vicious lizard in the living room. In the backyard, Alexander sees his parents had rented an Australian petting zoo for his birthday party. During the party, he learns that Paul and Becky have shown up.
During the credits, they are seen pretending to surf.
