The Ugly Truth begins with Greg discovering that Rowley has a new friend that his parents hired from a company called "Cool Brian". Left friendless, Greg attempts to find another friend and become popular on his own, only for this to backfire on him. At the same time he's dreading his uncle's impending wedding, knowing that his grandmother "Gammie" will try to have The Talk with him. Things are further exacerbated by his mother going back to school, which means that Greg, his father, and his brother will be left to care for themselves. Rodrick takes advantage of the situation to play several pranks on Greg, which culminates in him accidentally setting off a fire alarm at school, but manages not to be caught.
As time progresses, Greg is subjected to more misery by way of an ill-tempered and paranoid dentist, receiving a failing grade on an important Health class assignment, and dealing with an extremely lazy maid called Isabella. To make matters worse, his mother is initially unwilling to believe that Isabella is lazy until she discovers this for herself and fires Isabella as a result. Greg also gains more stress over the impending school "Lock-In", where the boys and girls have a supervised slumber party in the school auditorium. The slumber party turns out to be a disaster, as the girls and boys are separated and the games are all either childlike and boring or risky. The slumber party is made even worse when one of the chaperones turns on the air conditioner to keep the misbehaving boys in their sleeping bags. The next day Rowley comes to school with a large zit on his forehead as a result of his growth spurt, which he believes will make him cooler and more attractive to girls. This leads Greg to question his parents over his own lack of a growth spurt, to which they reply that they were both late bloomers when it came to puberty. Greg is briefly excited at the chance to attend a party by one of the popular kids, only to be crushed when he discovers that he is unable to go since the party is on the same night as his uncle's wedding. Disappointed, Greg attends the wedding, where he's bored and humiliated. He's later pulled aside by his Gammie, who gives him "The Talk", where she explains to Greg the stress and horrors of becoming and being an adult both physically and mentally. Gammie tells Greg to enjoy his late childhood for now. Normality eventually resumes when Greg's mother stops attending college and returns to the family and Greg discovers that the party invitation was a pretense to use himself and Rowley as servants. Greg also decided to make the first move and mends his friendship with his ex-best friend Rowley, and the two became friends again.