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

When they take their dog, Watch, for obedience training, the Alden children discover that some of the dogs in the class have vanished and decide to try to solve the mystery of their disappearance.

When the Aldens help a friend of their grandfather's to build the world's largest yo-yo, they soon discover that more than one person is trying to prevent them from finishing the job--and for more than one reason.

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

The Aldens discover that one of Pikesville's little league coaches has been cheating all season, which is somehow related to a plan to raze the town's historic baseball field.