As the Heffleys embark on a fun-filled trip to the local pool, they unexpectedly cross paths with Lenwood Heath, a reformed delinquent linked to Rodrick who has found his calling as a lifeguard. This chance meeting sets off a chain of events that greatly affects Greg, prompting Frank to consider enrolling him in the highly disciplined Spag Union military school as he prepares to enter eighth grade.
Greg's summer begins with him spending an entire day in front of the screen, lost in video games. In response to this lapse, Frank resorts to extreme measures by confiscating Greg's gaming console, eager to spend some quality father-son time together. However, their bonding efforts fall flat, leading to frustration on Greg's part when Frank's attempts at fishing trips and American Civil War reenactments prove to be a disaster.
Just when Greg thinks he's stuck in a rut, Rowley invites him to join his family at a prestigious country club, where he hopes to impress his crush, Holly Hills. Although initially hesitant, Greg's interest piques upon discovering that Holly is a tennis instructor there. Meanwhile, Frank has different plans for his son, surprising him with an unpaid internship at his office, a well-intentioned but misguided attempt to forge a stronger relationship.
Feeling the pressure from his father’s expectations, Greg devises a scheme to fake a job offer from the country club, which he successfully uses to impress Frank and Susan. In a twist of irony, they reward him with a "Ladybug" cell phone, which is limited to home calls and emergency 911 calls only.
Longing for freedom, Greg eagerly accepts Rowley's invitation to their beach vacation. However, he soon becomes overwhelmed by the Jeffersons' dominant presence. In a desperate attempt to escape, he tries to email Susan using Robert Jefferson's laptop, but accidentally sends the message to his entire contact list instead.
As chaos ensues, Greg's situation takes another turn when he accidentally dials 911 while trying to reach home. This unfortunate mistake nearly leads to Robert getting arrested after he answers the door with a knife, convinced there might be an emergency. Following this incident, Frank begins to worry that Rowley is a bad influence on Greg, leading to tension between the two families.
As the summer continues, Greg's country club adventures reach new depths. Rowley is compelled to disinvite Greg from the country club, but with resilience and creativity, Greg sneaks in by impersonating club members. His latest scheme is fueled by Rodrick's revelation of a crush on Heather, Holly's older sister, who works at the club. Together, they manage to score a gig for Löded Diper at Heather's upcoming sweet sixteen.
As tensions mount, Frank discovers Greg has been indulging in extravagant smoothies that cost $260, leading to an explosive confrontation. The situation comes to a head when the manager reveals that Greg is not an actual employee of the club, leaving Frank feeling betrayed and disappointed.
The arrival of a Spag Union disc at their home sends Greg into a spiral of anxiety. Rodrick encourages him to attend the "Wilderness Weekend" camping trip with his classmates to win back Frank’s approval. During the camping trip, Fregley shares a chilling story about a mysterious muddy hand, contributing to Greg's growing unease. In a surprising act of accountability, Greg strikes Frank's hand with a mallet, indicating his willingness to own up to his mistakes.
The following day takes a turn when Greg overhears Stan Warren, their troop leader, belittling him. In retaliation, Greg concocts a plan with his friends, only to uncover that Stan's troop has been cheating. When Stan catches wind of Greg's stratagem, it leads to a moment of embarrassment for him, while Frank takes the opportunity to chastise Stan for his lack of competency.
Ultimately, a heartwarming conversation between Frank and Greg at a motel reveals their hidden similarities. They realize that they have more in common than they thought, with Frank admitting his distaste for camping. This validation paves the way for Greg's triumphant entrance at Heather's sweet sixteen, where Löded Diper performs a heavy metal cover of "Baby" with Rodrick leading the vocals. However, the performance spirals into chaos when Rodrick accidentally destroys a massive ice sculpture, resulting in disarray among the partygoers.
Heather’s attempt to retaliate using a microphone stand ends in calamity as she inadvertently wrecks a chocolate fountain, bringing the celebration to a startling finish. Amid the pandemonium, Holly begins to connect with Greg over their shared love for music, and their relationship flourishes alongside Rowley and the rest of the Heffleys throughout the summer.
