The Aldens try to help a visiting Russian gymnast who is surrounded by mysterious people and events and who seems to have a secret.

Six children help solve the mystery of a missing spyglass that has kept two brothers feuding for years.

The Aldens take part in a hunt at a gem mine and discover that someone is out to sabotage the contest.

While on bicycle trip to their aunt's farm, the four Aldens help solve a mystery.

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

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 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 Henry, Jessie, Violet, and Benny help a friend fix up an old mansion filled with skeletons belonging to the doctor who once lived there, statues begin disappearing from the mansion grounds and a walking skeleton may be the culprit.

The Aldens investigate when exotic and endangered animals begin to disappear from the zoo where their new friend Lindsey works.