The Alden children, isolated in a storm at a small bus station, are led into a mystery centering on a polluted river.

Though scoffed at by Harold the dog, Chester the cat tries to warn his human family that their foundling baby bunny must be a vampire.

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 solutions to ten mysteries solved by Leroy "Encyclopedia" Brown are given in a separate section challenging the reader to match wits with the ten-year-old mastermind of Idaville's war on crime.

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.

Cam and her friend Eric chase the thief of Aunt Katie's wallet through an amusement park and find themselves involved in another case requiring their special detective skills.

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.

The Alden children have another mystery to solve when they visit a castle and must figure out which of the guests has stolen the Stradivarius violin.

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

The Aldens become involved in a community theater production of "The Wizard of Oz," which is systematically and mysteriously being sabotaged.