When their grandfather buys a bookstore at auction in New Orleans, the Alden children help clean it up and discover that several people seem to be obsessed with the store and its contents.

The Aldens try to solve a series of mysteries which took place in a old hotel in Kansas which they visit.

While visiting Lake Lucille in the Adirondack Mountains, the Aldens hear stories about a monster in the lake and then encounter evidence that it may be real.

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

While visiting a Navaho Indian reservation in New Mexico, the Aldens try to save a forest under threat of development by proving it is the site of a lost, buried village, but someone is sabotaging their dig.

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

The Aldens investigate the rumors of ghosts, disappearing props and a haunted seat in the Trap-Door Theater.

The Boxcar Children go to summer camp, where they suspect that someone is cheating during the Camp Olympics. One warm night four children stood in front of a bakery. No one knew them no one knew where they had come from. Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they're spending a week at Camp Seagull on beautiful Claw Island. When they arrive at Camp Seagull, the Alden children...

The Aldens participate in a three-day bicycle ride and soon find that someone seems to be intent on keeping others from completing the event.

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.