In the 1930s, Matthew "Moose" Flanagan and his family move from Santa Monica to Alcatraz Island when his father takes a new job as an electrician and a guard in the well-known Alcatraz prison. Moose becomes friends with the warden's daughter, Piper, who regularly gets into trouble in her attempts to earn money to get off of Alcatraz. Piper talks Moose into being part of her money-making schemes, like having inmates on the island do laundry for the kids at school. When the scheme fails and the Warden receives word of it, the children are punished and have to find a new way to spend their time. In an attempt to gain acceptance, Moose hangs around the prisoners' rec center in hopes of finding a stray baseball for use in games with the other kids. Moose eventually notices his older sister Natalie developing a relationship with convict 105, also known as Onion, who is trusted and able to roam freely because his sentence is almost up. Onion knows Moose has been looking for a baseball and gives him one. Scared of his sister hanging out with a convict, Moose is only reassured because of his confidence that she will be re-accepted to the Esther P. Marinoff School for people with special needs. Moose and his family's hopes are crushed when the school rejects Natalie. Desperate to help Natalie, Moose, with the help of Piper, writes a letter to the infamous criminal Al Capone, who works in Alcatraz's laundry. The letter asks Capone to pull any strings he has to help Moose's family get his sister back into school. Within days, Natalie is accepted into a new Esther P. Marinoff School branch for older children. The next day, Moose is getting ready for the day when he finds a note in the sleeve of his shirt with the word "Done" underlined.