The world is beset by the appearance of monstrous creatures, but one of them may be the only one who can save humanity.

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.