Mason Vox was the provost of Darschon. He was the father of Cilia and an old friend of the Doctor's. When the Fourth Doctor arrived on Agratis, finding Judoon on the planet, he went to Mason. Mason told the Doctor that the Jewel of Fawton, the main source of economy for Agratis, had been stolen.
When Cilia told Mason that she thought Roget may have stolen the Jewel, the Doctor accompanied Mason and Curator Frez to the chambers of the Krytuk Valleys, where the Jewel originated, while Leela and K9 Mark I distracted the Judoon. Inside the chambers, the group discovered Roget. He had placed the Jewel, which was the hive mind for the native crystalline species, in its original place, and awoke the creatures.