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 an elderly friend is injured, the Alden children offer to help him with his window-washing business in their small town of Greenfield and agree to help him solve a riddle that has arrived in a mysterious invitation.

When the Alden children go camping on Blizzard Mountain, a place said to be inhabited by a ghost looking for buried treasure, they begin to think someone or something is trying to scare them off.

When their favorite author fails to arrive as scheduled to speak at the local library, the Alden children investigate how he could board a train and never be seen again.

Someone or something is making a mysterious tapping sound at midnight in the home of a famous inventor known for her one-of-a-kind clocks. There is an invention convention taking place in the home and the Alden children are competitors at the convention. Does the mysterious tapping mean trouble for the convention? The children are determined to find out.