The boxcar children travel to Maine, take canoes into the wilderness and live in a house exactly like the one on Surprise Island, except the Maine house is brown, not yellow. They untangle the mystery of the two houses. Relates the exploits of the four Alden children as they locate a missing man. The Boxcar children are determined to solve the mystery that surrounds an old yellow house on their grandfather's property. Strange things have happened...

Grandpa Alden's sister, Jane, is not feeling very well, and it's the Alden girls who go to care for her. Soon a mystery begins to unfold and the clues are everywhere; from the stranger on the train, to the beautiful yellow rocks on Aunt Jane's ranch. When Henry and Benny come out to join the girls, the four children find themselves unravelling the intriguing mystery.

Henry, Jessie, Violet and Benny share a mysterious island for the summer with their grandfather.