Moose Flanagan and his family move to Alcatraz Island because his father has been hired as an electrician on the notorious prison island. Meanwhile, both his parents hope that his sister, Natalie, will now have a fair chance to be accepted to the Esther P. Marinoff School in San Francisco.
Although Moose is only twelve, he is extremely protective of his older sister, who turns sixteen during the course of the story. When Natalie is rejected for attendance at the school after only 36 hours, Moose's parents are dejected and frustrated. The principal of the school, Mr. Purdy, recommends that they work with Mrs. Kelly to get Natalie ready for the program at the Esther P. Marinoff School. Now, at Mrs. Kelly's recommendation, Moose has to take his sister with him everywhere he goes. Although Moose is initially very angry with his mother and Mrs. Kelly, he buckles down and tries to accommodate Natalie in his daily life.
Moose takes on the responsibility of watching his sister despite his inexperience and personal frustrations. Although some people would say that the challenge is unfairly deposited on his young shoulders, Moose shows compassion and maturity in the way he handles a hysterical Natalie during one of her tantrum episodes. He is gentle and firm with her, wrapping her in a rug until she feels secure. Despite his frustrations, he knows that Natalie cannot protect herself either mentally, emotionally, or physically. When he finds out that prisoner No.105 has been sitting and holding hands with Natalie, he is frantic with worry, yelling at the convict to get away from his sister. Although he is terrified of the consequences when Natalie meets with the convict a second time, he continues to watch over his sister. Moose is both loyal and practical, proud that his sister has a chance to act like a normal teenager (although her choice of admirer leaves much to be desired), yet worried about her safety.
Moose's honesty compels him to beg his mother to stop pretending that Natalie is ten years old. Although he knows that he will face his mother's anger, he courageously tells his mother the truth: that it would be far worse to pretend that Natalie is ten than to be honest about her real age. Despite the prejudices against mentally challenged older children, Moose reasons that Natalie will have a better chance of getting into the school if his parents are honest with Mr. Purdy about her age.