The film follows the exploits of Alexander Cooper (Ed Oxenbould), an ordinary 12-year-old boy, and his "terrible, horrible, no good, very bad day." He is left out by his family; his older brother, Anthony (Dylan Minnette), his older sister, Emily (Kerris Dorsey), his mother, Kelly (Jennifer Garner), his father, Ben (Steve Carell), and his baby brother, Trevor (Elise/Zoey Vargas), living in the Los Angeles metropolitan area of California. Anthony is trying to earn his driver's license so he can drive his bitter girlfriend, Celia, to the prom; Emily is rehearsing for the title role in her school play; Kelly is working for a publication company that is publishing a new children's book which will promote her to vice president of the company; and Ben, who has been unemployed for seven 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 mishaps, such as accidentally setting fire to his crush, Becky Gibson's, notebook as well as finding out that his friends, including Becky, and even his best friend, Paul, will all be attending the birthday party of Philip Parker (Lincoln Melcher) instead of his, due to Philip's expensive party entertainment and popularity. That night, Alexander puts a candle on a sundae and wishes that everyone in his family would experience what he is going through..