Henry, Jessie, Benny, and Violet Alden meet Mike Wood and a strange chain of events begins. Mike's house is mysteriously burned down, a man in a blue hat appears, and the Aldens' uranium mine is in danger of being sabotaged.
Four orphaned children, afraid to go live with their grandfather, set up housekeeping in an abandoned boxcar and are doing fine until one of them becomes ill.
The Boxcar Children, spending the summer at Mystery Ranch with the Aunt Jane, try to help when their friend Mike Wood's house burns down and he is blamed for starting the fire.
The Aldens discover that one of Pikesville's little league coaches has been cheating all season, which is somehow related to a plan to raze the town's historic baseball field.
The Aldens stumble upon a new mystery to investigate when they discover that the fruits and vegetables they are helping grow in a local restaurant's garden are disappearing.