When the Bob-Whites--minus Dan--head to Cobbett's Island to spend ten days in a historic home on the beach, Trixie finds an old letter tucked away in a book and sets off on a long-delayed treasure hunt.

Thirteen-year-old Trixie Belden and her friend Honey make several trips to the local marsh in search of herbs and flowers to replace their teacher's plant specimen collection and to look for the pirate treasure that is rumored to be hidden there.

While preparing for a fund-raiser antique show, Trixie and the other members of the Bob-Whites discover a secret code in the pages of an old magazine in the attic.

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.

Trixie suspects her friend's butler of being involved in the theft of a priceless Oriental vase.

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.