Thirteen-year-old Trixie Belden and her friends search for hidden treasure in a mysterious mansion owned by an old miser.

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 vacationing at her uncle's sheep ranch, Trixie and the Bob-Whites track down the thieves who are behind the mysterious disappearance of several sheep.

Not long after her family becomes wealthy, Diana Lynch's long-lost uncle appears and starts ruining her social life, causing Trixie Belden to investigate whether Uncle Monty is an imposter out to steal the Lynch fortune.