While her father is looking for a homestead in the Dakotas, Addie and her mother and brothers spend the summer with her aunt and uncle in Sabula, Iowa, where she rescues her best friend during a flood and where her baby sister is born.
Twelve-year-old Addie eagerly awaits her cousins' summer visit to her prairie home but, once they arrive, finds things much more complicated than she had ever imagined.
Trying to be detectives, Mary Lou and her friends imagine that terrible crimes are being committed when Leroy, the worst kid who ever lived on Eighth Avenue, returns to the house next door.