Darkover was experiencing a flare-up of Trailmen's fever, an episodic disease that would decimate the entire human population of Darkover, from the Comyn to the Terrans. 

The Medical Branch at Terran HQ had the start of a cure, but in order to finish it, they needed Trailmen to come out of their homes in the trees in the Hellers Mountains and donate blood. 

Only one man on Darkover stood any chance of persuading the Trailmen to help, but he occupied the same body as the doctor capable of doing the medical side of the work, and he was the personality the doctor had utterly suppressed. 

Even with hypnosis, only one of them could be active at a time, and the solution would need both of them.

Lew Alton was returning to Darkover - returning at the command of men who had once been all too glad to see him leave. 

For Lew, a Darkovan on his father's side, and a Terran on his mother's, had always walked between two worlds, accused by each of belonging to the other, and trusted by neither. 

Yet Lew alone had the power to understand both worlds and to save them from each other's unknown forces. 

That was the reason he had returned at last - armed with the legendary sword of the Sharra matrix, whose destiny was to cross forces with the equally mystic Sword of Aldones in one mighty battle that would decide Darkover's fate