One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from. Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and a young Russian gymnast has come for a visit. Henry, Jessie, Violet, and Benny are happy to share their home with Katya, the young gymnast, and to watch her practice for the big competition. When things start going wrong at the gym it quickly becomes clear that someone is trying to ruin Katya's chances of winning. But who? The Aldens want to help. The trouble is, Katya's been acting a little mysterious herself — almost as if she's got something to hide. But what? Can the Boxcar Children solve both mysteries before the big competition?