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 in that house where long ago a man disappeared.

A family starts out on a one-day mountain climbing excursion that becomes very exciting. In fact, it becomes an adventure for the former "boxcar" family. They unearth some mysteries and have a fine time in the process.

The Alden family rents a houseboat to drift down the river, and encounters a mystery in the little towns along the river, involving a fast black car.

The Alden children, isolated in a storm at a small bus station, are led into a mystery centering on a polluted river.

The children are spending the summer on their own private island but there is a mysterious stranger on the island.

The entire town of Yellow Sands is left puzzled by a mysterious fire.

Grandpa Alden and the Alden children encounter danger, excitement, adventure, and mystery in a quiet island fishing village.

The Alden children save an old woman's house, land, and all the animals in her animal shelter from greedy contractors.

At an old library, the Alden children discover a boy who needs their help and a stranger who is after a valuable object they found in the library.

While on a camping trip with their grandfather, the Alden children try to find out about the loud music and missing food that threatens to scare away other campers.