Henry, Jessie, Violet, and Benny are going on a train trip across the country with their Aunt Jane! They love exploring the train, watching the beautiful scenery from the observation car, and eating in the dining car.

The Alden children witness something unusual on the train platform before they leave Boston for San Francisco with their aunt. A girl bringing an art portfolio on board snatches it from an older woman, knocking the woman to the ground. They attempt to solve the mystery aboard the train, but it leads to an even bigger adventure. It's a mystery that is certain to satisfy many elementary readers. It has just enough action without being too threatening or life-endangering for the characters. ( )