When Greg Heffley begins his last year of middle school, his mother gives him a journal to write and draw in, and this book is the product of that year’s recordings. Greg begins his journal by describing the other students and analyzing middle school popularity, a quality he will seek all year. He tells the story of an old slice of cheese left outside on a basketball court. If someone touches the cheese, they are cursed and ostracized. Greg introduces his older brother, Rodrick, who plays drums in a heavy-metal band and plays merciless tricks on Greg; his younger brother, Manny, who is treated like a prince; and his best friend, Rowley. Greg is “super good” at video games even though his father is always encouraging him to spend more time outdoors. When Greg uses headphones to listen to Rodrick’s CDs that have parental warning labels, he gets caught and punished with a video-game ban. At school, Greg runs for class treasurer, but his inappropriate posters get thrown in the trash by the vice principal.

In October, inspired by a haunted house in town, Greg and Rowley build their own haunted house in Rowley’s basement. When one frightened child crawls under the bed, the boys get caught by Rowley’s father, who shuts down the haunted house and grounds Rowley for a week. Later, when Greg and Rowley go trick-or-treating, they get soaked by a group of high schoolers in a truck (known as the Whirley Street Kids). When Greg and Rowley threaten to call the cops, the teenagers chase them. They hide in Greg’s Gramma’s house, but when Greg’s mother insists they come home, Greg’s dad soaks them in the driveway.

In November, Greg’s physical education teacher announces a unit on wrestling, and the boys are paired by weight, which means that Greg has to wrestle another lightweight boy named Fregley. Greg wants to move up a weight class, so he approaches his parents for a bench press. But they want to wait to purchase one until Christmas. Greg’s mother makes him try out for the school play, The Wizard of Oz, and Greg is cast as a tree. The play ends up being a disaster. As Christmas approaches, Greg asks for the video game Twisted Wizard, and the family buys a red sweater for a Giving Tree program at church. On Christmas morning, Greg receives a bench press, but he doesn’t care about it anymore. By accident, Mom also gives Greg the red sweater meant for the Giving Tree. Greg gives Rowley a L’il Cutie book he got (but hated) for Christmas, and Rowley gives him a child’s Big Wheel. When Greg teases Manny on New Year’s Eve, he gets sent to bed early.

In January, Greg and Rowley invent a game for the Big Wheel in which Greg throws a football at Rowley as he rides down a hill. Unfortunately, Rowley breaks his hand and gets lots of sympathy and attention at school. Greg signs up for Independent Study, and his class is assigned to build a robot, but the class gets canceled when the boys make a list of all the words that the robot should not say. Greg and Rowley sign up for Safety Patrol to walk kindergarteners home halfway through the day. In February, it snows, and Greg and Rowley try to set the world’s record for the tallest snowman, which ends with an ambush by the Whirley Street kids. When the school paper needs a new cartoonist, Greg and Rowley create cartoons that end with “Zoo-Wee Mama!” Greg creates his own character called Creighton the Cretin, submits several samples, and wins the position. However, Mr. Ira revises Greg’s masterpiece into a comic that promotes math.

In March, Greg chases the kindergarteners with a worm on a stick, and Rowley gets blamed. When Greg feels confused about whether to confess, Rowley tells the truth. Greg gets kicked off the Safety Patrol and Rowley gets a promotion. In April, Rowley gets a new best friend, Collin Lee. When Rowley has a sleepover with Collin, Greg goes to Fregley’s house for a sleepover, but it ends badly. It is during this time that Greg decides that he wants to be named Class Clown.

In May, when his history teacher announces that they will have a substitute, Greg considers it the perfect opportunity to play a trick, but his plan fails when his mother shows up and embarrasses him. Rowley becomes the new cartoonist for the school paper with “Zoo-Wee Mama!” When Greg confronts Rowley about stealing his idea, the former friends nearly fight on the playground. The teenagers from Halloween show up and make Rowley eat the cheese from the basketball court. The next day, when someone notices that the cheese is gone, Greg takes the blame. In June, Greg and Rowley start hanging out together again. On the last day of school, when Rowley is named Class Clown, Greg throws his yearbook in the trash.