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

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.

While staying at a hotel in New York City, the Aldens investigate a series of annoying pranks plaguing the management and guests.

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

When the Aldens go to the Pickering Natural History Museum to assist with the opening of a dinosaur exhibit, their work is hampered by a series of mysterious happenings.

While on a cruise with their grandfather, the Alden children help discover who is behind the mysterious troubles of the ship--disconnected phone lines, engine trouble, a man overboard--and still have fun sightseeing, eating, and swimming.

The Aldens try to help a visiting Russian gymnast who is surrounded by mysterious people and events and who seems to have a secret.

When the Alden children go camping on Blizzard Mountain, a place said to be inhabited by a ghost looking for buried treasure, they begin to think someone or something is trying to scare them off.