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 are spending the summer traveling on a train--in their own caboose, Number 777.

At one train stop, a man startles the Boxcar Children when he runs away after hearing the number of their caboose. It isn't long before the Aldens learn the mysterious history behind caboose Number 777--a history that involves a circus clown, a hisgh wire artist, and a valuable necklace. Come aboard and join the Boxcar Children in the Caboose Mystery