Greg Heffley finds himself in his final year of middle school, where he is beginning to go through puberty. What really gets him thinking about growing up is when he is assigned to treat an egg as if it were a baby (as seen in the cover). Greg is also realizing that Rowley might never understand the concept of growing up, and this ends up getting them into their biggest fight ever. The book will decide if they remain friends. The book also features Trista (who is seen at the end of The Last Straw, and the beginning of Dog Days), as a major character, and Greg's new girlfriend.