The main characters in the story are Greg Heffley; his little brother; his older brother, Rodrick; and his friend Rowley.

The story takes place over the summer and in Greg's hometown. He struggles to have a great summer at home, but problems seem to follow. The main problem is that Greg's mother, Susan, wants the boys to have "the best summer ever"—but it turns out to be the opposite for Greg.

Greg tries a few things: He starts a lawn-mowing service, but he doesn't know how to mow. He tries helping out a crush, a lifeguard at the local pool, but she expects him to clean up vomit, and he is not having it. He really wants a dog, but when they finally do get one, it turns out to just bark a lot and lick Greg's pillow. And he goes on a trip with his friend Rowley to the beach, but that turns out horrible, too.

In the end, he tries to end the summer on a happy note and enter a video game competition, but he scares Rowley into crushing his hand, and he can't even compete. So, it turns out to not be "the best summer ever" for poor Greg.