As a trained apothecary – so skilled at her job that her facilities even impressed T'Pol despite their primitive standards, Phlox reflecting that she would have been a fine physician if she'd been born on Earth or Vulcan – she was involved in researching a mysterious disease that killed her brother. She believed that a curio shop run by Garos was the source of the epidemic that was spreading among her people. She and a disguised Archer, who had arrived on the planet to explore and observe its pre-industrial civilization, discovered that Garos was really a Malurian whose mining operations contaminated the planet's water supply.
They were successful in driving off Garos and his cohorts, and after removing all of the mining equipment, the Enterprise crew provided a cure for the epidemic.
Riaan and Archer had a brief romantic relationship before the latter left the planet, Archer assuaging her fears at seeming backward to the space-faring Humans by noting that without their technology they were really rather similar. (ENT: "Civilization")