The story takes place on the planet Darkover, a world colonized by humans long ago, which has since developed its own unique culture and societal structures, blending Terran and native elements. The plot centers on a recurring plague known as Trailmen's Fever, which threatens both the human and native populations of Darkover. Dr. Randall Forth, a prominent Terran physician, is tasked with finding a cure for the fever. His efforts are complicated by the fact that the Trailmen, the native mountain dwellers who may hold the key to the cure, are reclusive and difficult to approach. Additionally, the terrain and climate of Darkover pose significant challenges to the research team. Dr. Forth's solution involves enlisting Dr. Jason Allison, a psychiatrist and physician with a unique condition: he suffers from a split personality, with one persona being a man of action, and the other a reserved, scientific type. Jason's alter ego, Jay Allison, is activated for the mission because his bold and adventurous nature makes him more suited to the dangerous journey into the Trailmen's territory.