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.
The Alden children are thrilled when a famous orchestra comes to their town to perform, and when an original Mozart score turns up missing, they solve the mystery of its disappearance.
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.
When their grandfather arranges for them to spend two week at Sunny Oaks, the four Alden children enjoy settling into the routines of farm life but become suspicious about a mysterious horse locked in the stables.
At the same time that the Alden children's efforts to raise money for a new hospital wing are being sabotaged by nasty tricks, they find their attempts to befriend the new girl Beth frustrated by her strange behavior.
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.
When the Alden children and their grandfather visit Everglades National Park in Florida to solve the mystery of a missing ranger, they discover that a panther provides a clue.
While the Aldens are visiting a friend of Grandfather's at the beach, Benny finds a long-lost piece of a map to a legendary pirate treasure, a discovery that brings unwanted attention from fortune hunters.
When the Alden children set out to solve a jewelry store robbery, they find they must also deal with a mysterious thief who steals their favorite cereal.
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 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.