A vastly superior alien race has boiled out of the galactic core, bent on expansion into Human and Sumerian space. Regent Cordez has ferried in the agile minds of the Mersa, and made contact with the elusive Druani, to create a new Alliance.

Research teams have leaped generations to find the technology they need, working out of the giant Prometheus base on Proteus, one of the moons of Neptune, but the Alliance is pushed back to toeholds on their key planets. With the Alliance in disarray the alien invaders set up bases in the streaming tails of fire that rip between the suns of binary systems.

Civil war rages across the medieval planet Hud as the Descendants of the Prophet are pushed back toward Roum, and Celia's research team crosses half the Spiral Arm in a desperate effort to solve a mystery that might put the Alliance back in the fight.