The narrative centers around Greg Heffley, a student at Westmore Middle School, who ingeniously chronicles his awkward and often hilarious experiences in his diary. This diary is intended for a future audience who will one day discover it when he is "rich and famous." As he delves into his chaotic life, Greg describes the local lore surrounding a moldy piece of cheese that has taken residence on the school's basketball court for what seems like an eternity. The superstition claims that if anyone dares to touch the cheese, they become afflicted with the infamous curse known as the "Cheese Touch," rendering them repugnant to their peers until they can pass it on by touching someone else. Thankfully, the last poor soul cursed by the Cheese Touch has since moved to California, and Greg fervently hopes that the curse does not resurface.
In his diary, Greg also dissects his relationship with his best friend, Rowley Jefferson, whom he depicts as somewhat immature and naïve. Greg feels that his insightful ideas often "go in one ear and out the other," illustrating the comedic yet frustrating dynamic of their friendship. At home, Greg faces daily bullying from his older brother, Rodrick, while also harboring jealousy towards his spoiled toddler brother, Manny, who seems to evade all repercussions for his actions. Unfortunately for Greg, his father does not support his penchant for playing video games; instead, he pushes Greg to engage in more physically active pursuits outdoors.
As Halloween approaches, Greg shares a rather amusing anecdote about his father, who enjoys lying in wait in the bushes on Halloween night to drench unsuspecting teens passing by their home with a trash can full of water. Inspired by the high turnout at Crossland High School's haunted house, Greg and Rowley decide to create their own haunted attraction. Unfortunately, their efforts yield only a mere two dollars. Their Halloween night takes a turn when they go trick-or-treating and encounter a band of teenagers who spray them with a fire extinguisher filled with water. Adding to their soaked misery, Greg's father once again mistakes them for rowdy teens when they return home and douses them with a trash can of water.
In his Geography class, Greg faces his own challenge when he flunks a test on state capitals, thanks to a reminder from a girl named Patty Farrell to the teacher about covering the map. At home, his mother unexpectedly pushes him to audition for the school play, which is an adaptation of The Wizard of Oz. Greg unexpectedly lands the role of a tree, while Patty takes on the role of Dorothy, the central character. When the performance rolls around, Greg's nerves get the best of him, causing him to forget the song and confuse the other trees. In a fit of frustration, Patty lashes out, and Greg retaliates by throwing apples at her, leading to the play's abrupt closure after Patty's glasses get destroyed. Despite his mother’s disappointment, Greg finds momentary joy in getting back at Patty.
As Christmas approaches, Greg finds himself disheartened by the lackluster gifts he receives. In an attempt to bring excitement to his otherwise mundane holiday, he engages Rowley in a reckless game involving a Big Wheel and throws footballs in an attempt to knock Rowley off. A mishap leads to Rowley falling and breaking his hand, which ironically earns him sympathy and attention from girls at school, much to Greg's envy.
Seeking to rise above his social standing, Greg joins the Safety Patrols at school, alongside Rowley, hoping to exert some authority over the other students. The perks of being in Safety Patrol include delightful treats like free hot chocolate. Greg aspires to become the school's cartoonist and partners with Rowley to create a comic strip titled Zoo-Wee Mama!. However, as he becomes more ambitious, he seeks to branch out with different ideas while Rowley remains intent on sticking with their original concept. Despite his efforts, Greg's version of the comic faces significant changes that undermine his creative vision, turning his character into a "curious student," a far cry from the quirky "cretin" he intended.
A pivotal moment comes when Greg is mistaken for Rowley after an incident involving kindergartners—and the resulting chaos leads to Rowley’s suspension from the Safety Patrols. Thankfully, the truth about Greg's antics comes to light, leading to Rowley’s reinstatement and a subsequent promotion, while Greg faces dismissal from the position.
As the school year nears its end, Greg sets his sights on being featured in the yearbook's Class Favorites page, particularly aiming for the title of "Class Clown." However, fate takes another turn when he discovers that Rowley is the new cartoonist and that his Zoo-Wee Mama! comic has remained untouched in its original form. An argument ensues when Greg confronts Rowley about not acknowledging him as co-creator, leading to the heartbreaking revelation that Rowley believes the comic was purely his idea.
In a twist of fate, they are confronted by the teenagers from Halloween, and the duo is coerced into eating the cursed Cheese. Cleverly, Greg claims to have a dairy allergy, allowing him to sidestep the curse, while Rowley is left to consume the whole piece. When word gets out that the Cheese has mysteriously vanished, Greg opts to take the blame for Rowley, asserting he disposed of it, ultimately allowing their friendship to mend. In the end, as Greg flips through his yearbook and sees Rowley’s picture on the "Class Clown" page, he tosses it in the garbage, thus dooming himself to carry the Cheese Touch into the next chapter of his adolescent life.
