Hattie Owen is just about to celebrate her twelfth birthday as the story opens. She says good-bye to her best friend, Betsy, who always leaves their hometown, Millerton, for summer vacation in Maine. Betsy invites Hattie along, but Hattie doesn't want to leave her family and home. Hattie's parents run a boarding house and Hattie is well acquainted with two of the boarders, Mr. Penny and Miss Hagerty. The third, Angel Valentine, is the newest boarder. She is very young and attractive.

As Hattie is settling into her summer routine, she learns that her Uncle Adam is about to return to Millerton. Adam is mentally challenged and has been away at a special school since he was very young. Hattie has never heard of her Uncle Adam. Hattie's Nana is upset that Adam is returning. Nana has definite ideas of what's socially acceptable and hates that Adam is unable to conform to those ideas. Adam remembers Hattie, having been sent away to school when Hattie was just a baby. Hattie soon learns that her mother had sent pictures and letters to Adam every week while he was away.

Hattie is curious as to why she never knew that Adam existed. She also wonders if her parents never had another child for fear that they would have a child with Adam's mental difficulties. She can't bring herself to ask outright. Hattie stands up to two girls who call Adam a "freak". She later says that this is only one of the challenges Adam faced during his life and that it made it difficult to be Adam.

One day a carnival arrives in Millerton. Hattie becomes friends with a carnival kid named Leila. Hattie spends many afternoons at the carnival and the girls ride all the carnival rides for free and eat whatever they want. Adam and Hattie attend the carnival together twice. Hattie feels that her grandparents are unnecessarily restrictive with Adam and helps him sneak out of the house to go to the carnival at night. When Adam loses control on the Ferris wheel, he's taken away by police and Hattie is in serious trouble with her family.

A few days later, Adam returns to Hattie's house. He is as happy as ever before and has flowers for Angel Valentine. Hattie goes to see if Angel is in her room and Adam goes along. He pushes past Hattie and opens Angel's door to discover that Angel is inside but that she and her boyfriend are locked in a passionate embrace. Adam rushes away and disappears. He hangs himself and his body is found late that night.

Hattie asks to speak during the funeral and finds the opportunity to speak out about Adam's good qualities. She especially points out that he was able to find happiness. Hattie soon befriends a girl who moves into the boardinghouse and discovers that her life can go forward and be better for having had Adam in it.