is the third book in The Boxcar Children series. In this story, the Alden siblings—Henry, Jessie, Violet, and Benny—discover a mysterious abandoned house on their grandfather’s property. Intrigued by its history, they learn that years ago, the house’s former owner, Bill, disappeared without a trace. The children set out to solve the mystery of Bill’s disappearance, uncovering clues that lead them on an adventurous journey. Their investigation reveals hidden secrets and ultimately helps them solve a family mystery, showcasing the Aldens' teamwork and resourcefulness.