Who is Matthew 'Moose' Flanagan from Al Capone Does My Shirts and what is their importance?

Matthew Flanagan, known as Moose, is a twelve-year-old boy when his family moves from Santa Monica to Alcatraz Island off of San Francisco. Moose has a sister who suffers from a mental disability. For this reason, Moose's mother encouraged her husband to find a job at Alcatraz as an electrician and prison guard so that their daughter might be able to enroll in a new school that is rumored to be able to help children like Natalie to function out in the real world. No one asked Moose if he wanted to move and he is homesick for everyone who was important in his life before now.

Moose struggles to fit in not only in a new school, but with the kids who have been living on Alcatraz for nearly a year. Not only this, but Moose finds himself in a position to cause his whole family to be homeless when the warden tells him any misdeeds on his part can cause his father to be fired without severance. To add to his troubles, Moose's sister is dismissed from the school and forced to work with a therapist to help her become better able to deal with the school environment. For this reason, Moose finds himself in charge of his sixteen-year-old sister every day after school. Now Moose must come home to babysit rather than play baseball with the other kids at school.

Life is more difficult for Moose Flanagan than the average twelve-year-old, but he manages to survive the tension and even learn from it. In fact, it is Moose's actions that help solve his family's most immediate problems.