Moose Flanagan, who lives on Alcatraz along with his family and the families of the other prison guards, is frightened when he discovers that noted gangster Al Capone, a prisoner there, wants a favor in return for the help that he secretly gave Moose.

Moose has his hands full during the summer of 1936 watching his autistic sister, Natalie, and the warden's daughter, Piper, and trying to get on a baseball team by proving he knows Al Capone.