The children, who are spending the summer in a lighthouse on the New England coast, are disturbed by a mysterious woman walking the lighthouse grounds. Jessie's dog Watch wakes up growling almost every evening at midnight, and Henry finds a puzzling note in the sand.

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!

Six children help solve the mystery of a missing spyglass that has kept two brothers feuding for years.

A young Canadian visitor helps the Alden family solve a mystery involving a lost coin collection.

Two brothers take summer jobs at a department store where a series of peculiar events brings them under suspicion.

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.

While on a bicycle trip to their aunt's farm, the four Aldens help solve a mystery.

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.