When Prosper and Boniface's parents die, their aunt Esther attempts to adopt the younger brother, an adorable five-year-old who eventually turns six as the movie progresses, named Bo (Jasper Harris). She plans to send twelve-year-old Prosper (Aaron Johnson) away to boarding school. However, before they can separate the two boys Prosper takes Bo to Venice, the magical city about which their mother often told stories.