Henry, Jessie, Violet, and Benny are happy to share their home with Katya, the young gymnast, and 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?