When anti-mutant Colonel William Stryker kidnaps Professor X and attacks his school, the X-Men must ally with their archenemy Magneto to stop him.

Greg Heffley is a 12 year old who is fresh out of elementary and transitions to middle school, where he has to learn the consequences and responsibility to survive the year.

The Heffley family attends a party at a roller rink, where Greg Heffley reunites with his best friend List of Diary of a Wimpy Kid characters#Rowley Jefferson, meets a new girl at school named List of Diary of a Wimpy Kid characters#Holly Elizabeth Hills who Greg instantly has a crush on, and reveals he passed on the Cheese Touch. During the party, his older brother List of Diary of a Wimpy Kid characters#Rodrick Heffley advises Greg to ask Holly to skate with him when Couples Skate begins and then interferes with Couples Skate by persuading a crowd of hooligans to enter the rink, along with replacing "Always (Atlantic Starr song)" by Atlantic Starr with a heavy metal music song. This prompts Greg's parents, List of Diary of a Wimpy Kid characters#Frank Heffley and List of Diary of a Wimpy Kid characters#Susan Heffley (who misunderstand this) to speak to Greg on the rink's public address system and then carry him off the rink. Greg, extremely angry with Rodrick for humiliating him, tries to tackle him, but he misses and in turn, lands in a little girl's birthday cake. She and her friends then furiously proceed to beat up Greg as the intro starts.

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.