The Heffley family gathers for dinner at Corny's, where Susan reveals a surprising announcement: they will be embarking on a road trip to celebrate Meemaw's 90th birthday in Indianapolis. This news is met with groans from Rodrick and Greg, who had been promised an enjoyable experience by their mother. Little did they realize, this journey would lead to a myriad of unexpected adventures.
As the evening progressed, Greg found himself in a rather humiliating predicament, getting stuck in a ball pit filled with diapers after being sent to rescue Manny. The resulting chaos would cause him to gain notoriety on the internet as "Diaper Hands." In an effort to disconnect from technology, Susan later confiscated everyone's phones, declaring the road trip a tech-free experience, a decision that wouldn’t stand the test of time.
During their night’s stay at a motel, Greg secretly retrieved his phone and teamed up with Rodrick as they plotted an escape to attend the Player Expo. Their main goal was to eliminate the embarrassment caused by the "Diaper Hands" meme. However, they soon crossed paths with two mischievous brothers dubbed the "Beardos," nicknamed for their father's impressive facial hair.
As the plot thickened, Greg discovered that his curiosity about a strange noise had disturbed the wrath of Mr. Beardo, who was not pleased with the damage done to his van during their rowdy visit. Tensions escalated when Greg inadvertently referred to them as the Beardos, calling Mr. Beardo a "Fat Beardo." Enraged, Mr. Beardo swore to seek revenge on the unsuspecting Heffley family.
Meanwhile, Manny’s luck seemed to be on the rise when he returned from a fair with an unexpected piglet companion. However, Mr. Beardo was determined to catch Greg, resulting in a series of unfortunate mishaps that almost sent the family off a bridge. In an act of mercy, they donated the piglet to a local petting zoo, disappointing Manny. The brief moment of relief quickly vanished as Greg manipulated their GPS system to lead them towards the Player Expo instead.
Back in their hotel room, the brothers devised a plan to surprise Frank and Susan with a gift for Meemaw, but their intentions were soon compromised as they made their escape to experience the Expo. Fate intervened when Greg found himself facing off against Mac Digby, a fierce contender in a gaming competition. In a misguided attempt to impress, his actions led to Mac’s screen crashing, culminating in a mortifying reveal of Greg’s alter ego, "Diaper Hands," by none other than Susan. Her desire for a relaxing family trip to connect fell flat, as her tone indicated her frustration with Greg’s attitude towards the family bonding experience.
Feeling overwhelmed by Susan’s controlling demeanor, Greg confronted her about her apparent naivety, prompting her to consider giving up her stringent rules about technology on their trip. Following the confrontation, Greg realized that Rodrick's video recording had captured him inadvertently, making their well-laid plans ineffective.
Upon resuming their journey, the Heffley family's motorboat was overtaken by a runaway tarp, scattering their belongings everywhere. In a twist of fate, the Beardos returned to take advantage of this disarray, reclaiming their stolen items as retribution. The Heffley family's pursuit of the Beardos ended when they hit a massive traffic jam. Undeterred, Frank decided to take an alternate route, leading them to the Beardos' motel where they successfully retrieved their property.
As the birthday celebration for Meemaw approached, Greg found himself adrift in the boat that had detached from their car. Despite the vehicle’s continued troubles, a kind tow truck driver, who spoke only Spanish, guided them back to the petting zoo to reclaim the piglet. By the end of this tumultuous adventure, Frank was gifted time off work to spend with his family, and Rodrick had funds to repair his van. Reflecting on the escapade, Greg expressed excitement for future adventures, only to have Susan surprise them with news of flying to their next destination.
In an amusing mid-credits scene, Greg finds himself snapping selfies with two girls who, recognizing him as the infamous "Diaper Hands," can’t help but chuckle at his unfortunate circumstances.
