A series of practical jokes at a restored nineteenth-century farmhouse makes the Aldens wonder if the ghost of its builder is at work.

The Alden children travel on a Mississippi paddle-wheel steamer to visit an old family friend in his cabin near Hannibal, Missouri, and try to discover who is responsible for the mysterious activities near the house.

When the Alden children accompany their grandfather to the Snow Haven Lodge, a series of odd occurrences threaten to disrupt the annual winter games.

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

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

The Alden children plan to ride in their boxcar at the Greenfield Founders' Day Parade, but the newly refurbished boxcar is stolen a couple of days before the big event ... with their dog, Watch, in it.

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.

Henry, Jessie, Violet, and Benny--four children who used to live alone in a boxcar and now reside with their grandfather--arrive at Camp Seagull overflowing with excitement, but they soon realize that some of the other campers are not above cheating in the annual camp Olympics.

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

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.