The first installment of Jeff Kinney's blockbuster Diary of a Wimpy Kid series offers a glimpse of the hilarious experiences to come in the life of crafty middle school student Greg Heffley. He begins his first diary by explaining that it is a journal and that he would not be pouring out his emotions in it. He says that writing in this "journal" was his mom's idea, however he explains why it will "come in handy" in the future: "The only reason I agreed to do this at all is because I figure later on when I'm rich and famous, I'll have better things to do than answer people's stupid questions all day long."
Greg comments on how he plans to increase his popularity level this school year, but his best friend Rowley seems oblivious to the fact that he doesn't look cool. Greg talks about the dreaded "Cheese touch" that a student can get if they touch a piece of molded cheese on the basketball court. The person with the Cheese touch can pass it on to another student who hasn't crossed their fingers. At home Greg tries to get away with slacking off, but his parents usually call him out and make him do things he doesn't want to. When he tried to get in lower academic classes, Greg suspected his mother spoke with the Principal to get him moved up. When he played video games after school his father kicked him out the house and told him to get real exercise. Instead of playing outside Greg goes to Rowley's house and plays video games with him. Greg's older brother Rodrick often plays pranks on him that Greg likes to use on Rowley. Rodrick is characterized as an emo teenager who has a heavy metal band named "Loded Diaper". Whenever Rodrick's band practices at home, Greg makes sure not to be there because they are "AWFUL."
One of the first things Greg attempts to do to boost his reputation at school is to run for Treasurer of the student government. He figured he wouldn't have any competition until he found out that Marty Porter, who is good at math, was running for Treasurer too. Greg made posters about Marty having head lice and being clumsy with the school funds, but the Vice Principal told Greg he was not allowed to "write fabrications about other candidates" and took the posters down. Greg figured his "political career" was over after seeing Marty handing out lollipops to students to get votes. Greg then sets his sights on a money making scheme after going to a high school haunted house and calculating how lucrative it is based on the ticket price. He recruits Rowley to make their own haunted house and charge kids in the neighborhood fifty cents a piece to get in. After the first kid gets too frightened by Greg and Rowley screaming at him and hides under the bed, Rowley's father shuts down their haunted house. When Greg and Rowley go trick-or-treating they end up getting chased by teenagers and hide out at Greg's grandmother's house. They taunt the teens while they are safely inside, but it turns out to be a huge mistake later on.
When a wrestling unit starts at school Greg is mortified that his wrestling partner is Fregley, a strange kid who lives in his neighborhood. He is disturbed by the fact that Fregley keeps pinning him and decides to gain weight to get out of Fregley's weight class. He asks his parents to buy him exercise equipment and his dad gets excited, but his mother tells him to do jumping jacks and sits ups for two weeks to prove he was serious. Greg makes his own makeshift exercise equipment by filling two empty gallon bottles with sand and attaching them on the sides of a broom stick. He calls Rowley over and makes him test out the weights. By the time Rowley finished bench pressing the "barbell" he told Greg he didn't want to weight train anymore. Greg ends up not lifting the weights at all. Just as Greg is settling in his ways at school his mother forces him to sign up for "The Wizard of Oz" school musical. He signs up for the role of a Tree assuming that he wouldn't have any lines, but the music director Mrs. Norton wrote an original song for the trees so everyone can sing in the play. At the time of the performance Greg chooses not to sing because Rodrick is there with a video camera and he doesn't want him to use the footage against him for the rest of his life. The musical is a disaster because everyone forgot their lines and many of the actors behaved inappropriately. Though Greg ended up having a good time, his family didn't enjoy the play.
Greg's next attempt to improve his status at school was to become a Safety Patrol. He enjoyed the perks of getting hot chocolate every school morning and missing most of Pre-Algebra when he had to walk kindergarteners home from school midday. Everything was going well until Rowley was accused of "terrorizing" the children and got in trouble. Greg felt guilty because he was the one who scared the children by chasing them with worms. When he confessed to Rowley that he was the real culprit, Rowley told the supervisor and Greg was dismissed from his Safety Patrol duties. Rowley stopped talking to Greg and got a new best friend. Greg then attempts to get in the Class Favorites section of the yearbook by becoming the school paper's new cartoonist. He submits his comic "Creighton the Cretin" and gets selected. Greg notices that no one is laughing at his comic strip. He finds out that his cartoon was changed to "Creighton the Curious Student" and the words were completely different from what he wrote so he quits being the cartoonist.
Rowley is selected as the new cartoonist for the school newspaper with his comic strip "Zoo Wee Mama." Greg confronts Rowley because they came up with the "Zoo Wee Mama" idea together before they stopped being friends. While they are arguing on the playground and others students instigate for them to fight, the teenagers who Greg and Rowley taunted on Halloween come and grab them. The teenagers force Rowley to eat the molded cheese. Greg tells them he's allergic to dairy to avoid eating the cheese. After the ordeal is over the students wonder what happened to the cheese. Greg tells them he got tired of seeing it and removed it. They claim Greg has the Cheese touch and stay away from him. Rowley starts talking to Greg again and things get back to normal. At the end of his first diary Greg is very disappointed when he sees that Rowley won the title of Class Clown in the superlatives section for his "Zoo Wee Mama" comics and wants Rowley to remember that he ate the cheese if he gets too full of himself.