Dog Days follows Greg Heffley's disastrous summer vacation, where his dream of playing video games is thwarted by his mom's push for family fun, leading to awkward beach trips, a pet dog named Sweetie, falling out with Rowley, and trying to impress Holly Hills, all while avoiding his dad's manly summer expectations, ultimately learning that sometimes unplanned adventures can be fun.

Dog Days (movie) blends plots from books 3 and 4, focusing heavily on Greg's summer, his video games vs. dad's bonding, his crush Holly Hills, and the new dog Sweetie, creating a more linear story with strong father-son themes (Greg's Dad Frank wants bonding, fears military school). The book is more episodic, featuring individual misadventures like the Lil Cutie Comics, the Beardo family, and the literal lost dog (Sweetie), while the movie streamlines these, adds country club scenes, and emphasizes Greg learning to appreciate his family, a departure from purely coincidental book endings and ending the chapter for this live action movie series