The Heffley family attends a pool party at the community pool, where the family meets Lenwood Heath, a former trouble-making friend of Rodrick who is now a model student after attending a prep school known as Spag Union. Greg's father Frank thinks about enrolling Greg there. Greg's summer vacation starts off badly: Frank bans video games in anger at Greg's laziness, and his mother Susan starts a book club for all the boys in the neighborhood, but she does not see their books as real literature causing the boys to never come back the next day. Frank is also getting jealous of the neighbor's athletic kids, and after his video game ban, Susan forces Frank and Greg to bond with each other: their bonding activities end in disaster.

One year after the Diary of a Wimpy Kid: Dog Days (film), while at the Corny's family restaurant, the Heffley family — consists of Greg Heffley, Rodrick, Frank, Susan and Manny — plans to take a road trip across the USA to attend Meemaw's 90th birthday. However, after Greg and Rowley rescue his younger brother Manny, who got stuck inside a tube in the play area, Greg ends up in a ball pit with a diaper stuck on his hand, startling him. He subsequently becomes famous after the people around him record him and post the footage on the internet, leading Greg to be dubbed as "Diaper Hands", much to Greg's devastation.