The book begins with Greg Heffley and Rowley Jefferson at the latter's country club. After a few weeks of going to the country club, Mr. Jefferson stops inviting Greg to come after Greg had annoyed him with complaints over the smallest things (like he was missing a little umbrella in his soft drink). Greg's mother, Mrs. Heffley, makes him take his little brother, Manny, go to the town pool. After going there, Greg wanted to go to the beach. Unfortunately, the Heffleys' could not afford to go to the beach.
Later, Greg and Rowley had a sleepover. They stumbled upon of Greg's older brother's, Rodrick's, horror movies. After the movie, Greg and Rowley become terrified of a muddy hand which was in the movie that strangled people. Unfortunately for Greg, Mrs. Heffley found out about the movie and made Greg start a reading club in which he was the only participant after only two days. The reading program, however, was put to a stop when Mr. Jefferson arrived with a huge bill of the drinks Greg and Rowley drank at the country club and forced the boys to pay it off. Greg attempted to pay it back by mowing lawns, but that didn't work out. Eventually, he had to use his birthday money to pay it off.
After Greg's birthday, Mrs. Heffley allows Rodrick and Greg to get a pet fish. Greg gets an angelfish and Rodrick gets a fish labeled "aggressive." A few days later, the Heffley's go to a water park since there is a family discount. It began raining shortly after and they had to go home. While they were in, Rodrick's fish ate Greg's fish. The next day was Father's Day. The Heffleys' went to Leisure Towers to spend Father's Day with Greg's grandfather. There, Mr. Heffley, tells how his dog ran away to a butterfly farm. Then Greg's grandfather reveals he actually ran the dog over and Mr. Heffley took off in disgust. He then bought a dog which Mrs. Heffley named "Sweetheart" and she shortened to "Sweety."
One day at a grocery store, the Heffleys' ran into the Jeffersons'. Mrs. Jefferson, unaware between the tense relationship between Rowley, Mr. Jefferson, and Greg, asks Greg if he would like to go to the beach with them. Before Greg can say no, Mrs. Heffley says that Greg would love to go. Greg is not used to how close the Jeffersons' are to each other like sharing one ice cream cone and playing charades. After a couple days, the Jeffersons' and Greg went to the boardwalk. There, Greg went on a ride called the "Cranium Shaker" after Rodrick told him that that was the only way to make him a "man." Rowley and Greg then wandered around the boardwalk before they were found by Mr. and Mrs. Jefferson who claimed they were going to call the police and report them missing even though they wandered for only an hour. After the got home, Mr. Jefferson made them go to their room. Rowley was very upset and as Greg put it, "I don't think he's ever been in trouble with his dad before." Greg then tries to cheer him up by using static electricity to shock him. Rowley then shocked Greg back playfully. Then Greg took a rubber band lightly hit him. Rowley then started screaming and his parents came and made Frank and take Greg away from Rowley.
Frank was very mad at Greg, not for hitting Rowley, but for making him pick him up in the middle of the night. Susan tried to stop this and made him take Greg to a baseball game. Frank tried to keep it a secret and on the way to the baseball stadium, Greg thought he was trying to sell him and called the police. After Frank got away from the police, he claimed he was not in the mood for baseball and went home disgusted in Greg.
Despite Greg's father being mad at him, they find a common good by looking at a horrible comic in the newspaper. Greg realizes everything is going to be all right between them when they rip on it.
The book ends with a photo album attempted to keep throughout the summer's activities but every single photograph was labeled the opposite of what the picture actually showed. However, he realises one fact: the person who takes the picture is the one who gets to tell the story.