Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather and are spending the summer out west with their Aunt Jane's ranch. Aunt Jane, their grandfather's sister, is a very unhappy woman who hasn't spoken to her brother in years. The boxcar children try to help her, but it isn't until the end of the summer that they learn Aunt Jane's secret and make a surprising discovery about Mystery Ranch.

Cataloging information taken from front/back of title page.

The door bangs -- The letter -- A day at the ranch -- The boys come -- Aunt Jane's nurse -- A strange offer -- The party.