When the Aldens spend the summer riding on a train, they get to live in a train car once again - this time, they're staying in Caboose No. 777. The number is hard to forget, and indeed, the children meet people at each stop who seem to know it well. It turns out Caboose No. 777 has an unusual past - one that comes with its very own mystery!

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

Relates the exploits of the four Alden children as they locate a missing man.

Recounts the adventures of the Alden children as they solve the mystery of the old schoolhouse in Port Elizabeth.

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.

The Aldens search for carousel horses that have disappeared from an amusement park.

While helping run Jerry Taylor's general store, the Alden children investigate his mysterious employee Nancy.

The Alden children investigate various mix-ups at a zoo run by their grandfather's friend in an effort to save it not only from pranksters but also from the town council.

The Alden children suspect someone of trying to sabotage the local dog show.

When their favorite pizza restaurant almost goes out of business, the Alden children try to help their friends stay open while they find out who is behind all their problems.