A group of boys chase after 12-year-old Willie, accusing him of being a freak. As he goes down the street, Willie notices a little girl, Charlotte, and approaches her. When she introduces herself, Willie realizes who her mother is. He walks her home and sees Charlotte's mother, who seems to recognize him as well. He talks about loving her once and then runs away past the bullies.
When he returns home, Willie writes in his diary that it's time for him to leave. His mother, Emma Webley, comes in and sees that he's packing. She isn't entirely surprised but is still hurt that he's leaving them. They go downstairs to talk with his father, John, and Willie tells them that he has to go. He then takes out photos of other families that he's stayed with, dating all the way back to 1963. In each photo, Willie is present and the same age. As he looks outside and sees Charlotte's mother walking by, he tells his current parents that he hasn't aged in the last 25 years, and that he has to move on when people realize he is always the same.
As he eats a final breakfast with his current parents, Willie says it's been a great three years and tells them he found them by looking for parents who had lost a 12-year-old son. He came to their home, claimed he was an orphan, and they took him in to replace their dead child.
Willie talks about his past, and how his birth parents died 25 years ago. Before he was sent to the orphanage, he played with Charlotte's mother, herself a child at the time. They were separated and at the orphanage, Willie realized that he had stopped aging. The other kids eventually realized that something was wrong with him when they grew older and he didn't, so Willie ran away. He tried to join a circus but the midget in charge, Tiny Tim, pointed out that they couldn't convince people to think of Willie as a midget, and there was no way to show he was a freak.
As Willie was walking in the park, he befriended an elderly woman who was crying. She bought him an ice cream cone and explained that her husband recently died and she lost her son 30 years ago. She tells Willie that she's seen her friends grow old and become tired, and how children grow old and become cynical and mean-spirited. She thinks it would be nice for children to stay young forever, and asks Willie to stay with her. Willie decides to cut himself from his past, remaining young forever. After three years, the old woman dies. Before she goes, she thanks Willie for staying with her and tells him to go and make others happy.
Willie decided that he would have to cut himself off from everything else and simply remain young. However, after three years the old woman passed away. As she lies dying, she thanks Willie for their time together and tells him to make someone else happy.
Willie finishes his story and says goodbye. As he goes to the train station, one of the bullies spots him. He throws a baseball at Willie, who catches it and throws it back at him, hard. Surprised at Willie's sudden display of skill, the bully throws it back at him, hard, and Willy catches it and throws it back even harder. The bully tells him to get out of town and runs away. Willie runs to catch the train. As he goes, he sees Charlotte's mother. She gets a good look at him and realizes that it's the boy from her childhood, unchanged after 25 years. Crying, Willie has no choice to get on the train. He soon arrives at a new town, checks the newspaper, and finds a new couple to take him in.