The film is based on the best-selling children's book series of the same name by American writer Jeff Kinney. On the first day of junior high school, Gray Hufflepuff (Zachary Gordon) was nervous and desperately wanted to stand out and become a star in the class yearbook. Gray's brother Roderick (Devon Bostick) gave him a lot of advice that he couldn't do this or that, but his good friend Rowley Jefferson (Robert Capron) didn't agree and continued to use the same dress, language and attitude as in elementary school, which gave Gray a headache. Gray rejected the girl Angie's (Chloe Moretz) invitation to join the school newspaper and pulled Rowley to join the school security patrol. When escaping from the three gangsters who offended Halloween, Gray made a mistake, but he lacked the courage to admit it and let Rowley take the blame for himself. Subsequent events developed worse and worse, Gray's popularity in school plummeted, and even his friendship with Rowley was in jeopardy...