Alcatraz Island in the 1930s
The setting kind of makes the story in Al Capone Does My Shirts. If Moose and his pals weren't stuck on a tiny island, they most definitely wouldn't get so creative—and if the island weren't full of convicts, they wouldn't have so many great opportunities to do so. And if it weren't the 1930s, Moose's dad wouldn't be desperate for a job (hey there, Great Depression), and they probably wouldn't have moved to the island in the first place.
Let's break it down for you.
Alcatraz was a high security prison, located off the coast of San Francisco in California. By high security, we mean it was home to some of the time's most dangerous criminals—guys who couldn't be trusted anywhere else. You know, like the infamous Al Capone, who was so powerful that he was able to weasel his way into getting special treatment at every other prison he was in. Adding to its security is the fact that it's an island, meaning for the kids who live on it, home is a pretty isolated place.
The fact that the story takes place during the Great Depression helps explain why Moose and his family end up living here, though. Remember Moose saying that his dad was normally an electrician? During the Depression, jobs in America were really scarce. Mr. Flanagan is lucky to have a job to support his family… even if that means sharing a tiny island with criminals.
Sometimes the setting isn't such a big deal, but in Al Capone Does My Shirts, you almost don't have a story without it.