The Heffley family goes on a summer road trip that quickly turns into a disaster. Greg is unhappy from the start because he feels trapped in the car with his family for a long journey.
As they travel, many problems occur: arguments, bad stops, car troubles, and embarrassing situations. The trip becomes even more chaotic when Manny adopts a baby pig from a petting zoo, which causes constant trouble during the journey.
The family also faces strange and stressful encounters along the way, making the vacation far from relaxing. In the end, the road trip is a complete mess, but it becomes a funny and memorable adventure for Greg.