Roberta (Bobbie) , Phyllis, and Peter lived happily with their parents. Until one day , the father was accused for betraying his company and had to be locked in prison (only Mother and Bobbie knows about this). The siblings and mother had to move to the countryside. Bobbie tried hard to convince an old gentleman in a train that her father was innocent and eventually succeeded. Their father went home with hugs awaiting him.
Besides , they were able to do a lot of things in the novel , including saving a train from crushing into a landslide, and got into trouble when Peter stole coal mines at night.
Hope this helps.