Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather and are taking a bus trip. They're bound for the Science and Hobby Fair, but when a bad storm hits, they're forced to stay in the bus station. And before long, the Boxcar Children are led into a mystery involving a polluted river, two mysterious boys, and a gruff bus station manager who knows more than anyone suspects...