Helen Plum Library

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

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

The Alden children hunt for their late grandmother's necklace that has been missing for years

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

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

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

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 investigate various mix-ups at a zoo run by their grandfather's friend in an effort to save it not only from pranksters but also from the town council

The entire town of Yellow Sands is left puzzled by a mysterious fire