While on a trip to St. Louis, fourteen-year-old Trixie Belden discovers some mysterious papers in her hotel room and soon realizes that she and her friends are being followed as they cruise in a towboat down the Mississippi River.

While vacationing at her uncle's lodge in the Ozark Mountains, Trixie and the Bob-Whites explore caves looking for the ghost cave fish, a rare species wanted by a national science magazine.

A child prodigy comes to stay with the Wheelers. Trixie and Honey go into the marsh to gather specimens for a school project. These two things add up to a mystery, of course.

Trixie walked over to her friend and grabbed her by the shoulders. "What is it, Honey?" she asked. "What happened?" "Oh, Trixie," Honey wailed, "I just got the most horrible phone call. H-He said I should call off the bikeathon. He--he said if I didn't--if the bikeathon went on as scheduled next Saturday--that something awful would happen. 'You can be sure of it, ' he said ..."

Trixie Belden and the other members of the Bob-White club visit her uncle's farm in Iowa, where they catch the thieves who had been stealing sheep in the area. 

When a strange writer haunts the Hudson River area near Trixie's beloved Sleepyside and her brother begins to manifest some mysterious and frightening symptoms, Trixie decides to investigate. Brian is poisoned and Trixie suspects his lab partner is responsible. Also, she spots a shark in the Hudson river! And then she meets an odd children's author who has a deep interest in sharks. What can it all mean?

Jim Frayne discovers he has a cousin, Juliana, an heiress to a small fortune and then she shows up in Sleepyside to claim it. meanwhile, Trixie and the gang befriend an amnesiac young woman called Janie whom somebody seems to want to harm.

The heavy front door closed solidly behind Trixie ... The long corridor that ran past the stairway was dark and gloomy-looking, and the stillness of the old building made Trixie feel she should walk softly ... She had almost reached the stairs when she saw a door open slowly. A tall man backed out into the hall and soundlessly closed the door. Turning, he saw Trixie ...

While trying to help a widow who grows flowers for a living, the Bob-Whites discover that an illegal planting of marijuana is the source of many of the problems that have been plaguing her.

A series of incidents which appear to be the work of a juvenile delinquent become a cause for concern for Trixie Belden and her friends-especially when Trixie's brother becomes a prime suspect.