Twelve-year-old Moose Flanagan moves to Alcatraz in January of 1935. He doesn’t want to be here, but Dad got a job as an electrician so that Moose’s sister, Natalie, can now attend the Esther P. Marinoff School. Natalie isn’t like other kids; at this school, a boarding school for disabled children, she can smear food in her hair and wear her clothes inside-out. Moose, meanwhile, is disturbed to be living on an island with notorious criminals like Machine Gun Kelly and Al Capone.
Over the next few days, Moose’s family settles in. Moose and Natalie meet seven-year-old Theresa and the warden’s daughter, Piper, who’s Moose’s age. Piper asks rude questions about Natalie, insists that “something is wrong with her,” and says her father won’t like having a “crazy” person around. The next day, however, Moose, Mom, and Dad take Natalie to the Esther P. Marinoff. Moose is conflicted: he wants what’s best for Natalie, and he isn’t sure this is it. But Mom is convinced this is necessary, so the family leaves Natalie with Mr. Purdy. When they get home, there’s a note summoning Moose to meet the warden. The imposing warden tells Moose the rules on Alcatraz (the most important being that Moose can’t talk about famous criminals like Al Capone to his classmates at school) and allows Piper to sit in on the meeting. Moose recognizes that Piper has her dad wrapped around her finger—she can get away with anything.
The next day is Moose’s first day at school. Piper blatantly goes against her dad’s rules and talks about Al Capone—and pressures Moose to do the same. This way, she insists to Moose, she and Moose can make a fortune selling the Alcatraz laundry service to their classmates; kids will be willing to pay them for Al Capone or Roy Gardner to wash their shirts (the convicts do all the laundry for Alcatraz’s civilian residents). Moose also meets Scout, a baseball player, and has a great time joining in on the weekly Monday game. After school, back on Alcatraz, Theresa introduces Moose to her brother, Jimmy, and Annie.
When Moose gets back to his apartment, he takes a phone call from Mr. Purdy: Natalie can’t stay at the Esther P. Marinoff, and Moose’s parents must come get her tonight. Mom is enraged and takes Moose with her. Mr. Purdy shares that Natalie screamed, implying that she threw a tantrum because he took her beloved button box away from her. He gives Mom the number for a woman named Mrs. Kelly, who might be able to help Natalie.
At school the next day, Piper ropes Moose into helping her with her laundry scheme. Over the next week, Moose’s life turns upside-down: Mom has met with Mrs. Kelly, and Mrs. Kelly believes that Natalie will improve if they take away her button box and let her hang out with other kids. This, Mom insists, means that Moose needs to look after her every afternoon—he can’t play baseball on Mondays. And though the laundry scheme does make Piper, Annie, and Jimmy money (Moose ultimately refuses to help and so Piper declines to pay him), a classmate’s dad writes a letter to the warden informing him of the scheme. The warden confiscates the money and scolds the children, and Piper goes to live with her grandmother in San Francisco for a few weeks. She insists she wants to be there, but Annie suspects Piper is being punished.
Over the next few months, Natalie integrates into the band of Alcatraz children. Moose lets her have her buttons every afternoon, and she doesn’t have “fits” anymore. Piper returns to Alcatraz suddenly in late March, ready with a new scheme: the kids are all going to take the boat to San Francisco and back on Sunday so they can catch a glimpse of Al Capone’s mother, who’s coming to visit. On Sunday, Theresa brings her new baby brother, Rocky, along. Rocky screams—until Mrs. Capone takes him and sings him a lullaby. The children are shocked and a bit disturbed. However, on Alcatraz, Mrs. Capone sets off the metal detectors and leaves without seeing her son. Moose later learns that she was strip-searched, and he feels awful for the poor woman.
The following week, Scout begins to talk to Moose about “convict baseballs,” baseballs that the convicts occasionally hit over the rec yard wall, and which Alcatraz children get to keep. He’d like one, and he implies that Piper is trying to find him one. Jealous, Moose begins taking Natalie to the bushes down the hill from the rec yard. She arranges rocks while Moose searches desperately for a ball. After the first day of searching, Natalie begins inexplicably saying, “105.” One afternoon, Moose returns to where he left Natalie—and she’s gone. He finds her with a convict, who offers Moose a baseball and then hurries away. The number on his shirt is 105. Piper catches Moose and Natalie as they’re getting back to their apartment, and she deduces what happened. She believes that with Natalie’s help, she can get Al Capone’s autograph.
Moose wants to tell his parents what happened, but neither of them are particularly concerned that a convict “noticed” Natalie. Mom is too focused on the fact that Natalie is improving (she’s using pronouns and expressing emotions for the first time) and doesn’t want to change anything about Natalie’s life or schedule. Though Piper tries to talk Moose into letting Natalie see 105 so they can spy on them and see what they’re up to, Moose refuses.
Moose spends the next week keeping Natalie inside in the afternoons so she can’t see 105. But one day, when Mom takes Natalie’s button box without telling Moose, Natalie throws a tantrum. Moose loses his temper too, but when he realizes that Natalie has no control and is effectively trapped in her own body, his anger disappears. He rolls her in the rug to help calm her down and when she’s calm, Natalie asks to go outside. They swing on the swings until Natalie falls asleep. Moose, with Dad’s help, carries Natalie home and cleans up the living room, and Dad assures Moose that Natalie’s condition isn’t Moose’s fault. That night, Dad tells Mom that they have to let Moose make decisions when he’s watching Natalie; the two of them have a relationship and they must respect that.
Since Natalie is interviewing for the Esther P. Marinoff School again next week, Mom is more nervous than usual. She frets about Natalie’s birthday the day before the interview—Natalie is, for the fifth year in a row, turning 10. On Natalie’s birthday, Natalie is anxious and seems on the verge of having a tantrum. But Moose takes her outside to walk, and Piper joins them. Piper holds Moose back while Natalie visits with 105 to say goodbye (he’s being released soon and he knows about the Esther P. Marinoff), and then they return to Moose’s apartment. Mom invites Piper to come to Natalie’s party, but ultimately, all the Alcatraz kids come. When Moose walks Piper home later, he reveals that Natalie actually is 16—and Piper says she figured as much. When he gets home, Moose tells Mom and Dad that they can’t keep pretending Natalie is 10. Mom is enraged, as she fears that nobody is going to care about a disabled adult—being a child forever is the only way Natalie will ever get help. But Dad asks Natalie how old she is. Natalie says she’s 16.
Natalie aces the interview the next day, and Mom apologizes to Moose for her behavior. Still, Natalie isn’t accepted to the Esther P. Marinoff. Moose realizes that night that Natalie has genuinely improved thanks to Mrs. Kelly’s suggestions, so he calls Mrs. Kelly the next morning to thank her. Since Mrs. Kelly believes the Esther P. Marinoff can help Natalie, Moose asks the warden for help getting Natalie accepted. When the warden refuses, Moose approaches Piper and asks for help sneaking a letter in to Al Capone with the censored mail, asking him if he can do anything.
A week later, Moose gets off the boat on the last day of school. His parents and the warden are waiting: Natalie got accepted to the Esther P. Marinoff; Mr. Purdy has suddenly decided to open a wing for older children. Moose feigns ignorance, but the following morning, he finds a note in the pocket of his freshly laundered shirt. It says, “Done.”
