They discover a Dyson Sphere under construction by microbots, but the bots are scavenging one part of the sphere to construct another. And where is the civilization that designed this massive project?

Braxton Thorpe and his crew, who originate from three different star systems, are on a voyage of discovery in the starship "Andromeda," headed toward the Cold Spot in the Cosmic Microwave Background, deep in the constellation of Eridanus. As they approach the Keid triple-star system, they stop at Keid A, around which rotates the planet Vulcan, home to Spock in the "Star Trek" television and film franchise.

Keid B & C revolve around each other, and both revolve around Keid A. While exploring lifeless Vulcan, Thorpe discovers that the planet is under attack by robotic scavengers. They follow the scavengers to Keid C, to discover a Dyson Sphere under construction by microbots scavenging one part of the sphere to construct another. "Andromeda" sets about exploring this immense sphere, half the size of an entire solar system.

Suddenly, the starship is attacked by those microbots, apparently seeing the starship as raw material. After narrowly escaping the bots, smaller spacecraft travel through the unimaginably immense sphere, studying how such a structure could exist, and searching for the builders. They discover several primitive races inside—humanoid, feline, and reptilian, each unaware that the others.

While they mount a study to learn what they can about these races, they discover a hidden population of electronic uploads stemming from the original builders of the gigantic sphere.

As the explorers begin to assemble a model of the sphere, they receive data showing the Dyson Sphere's star is about to produce a superflare that could destroy all life inside the sphere—and even the Dyson Sphere itself.

Can the explorers survive such an event? If so, can they save any of the sapient races, and will they succeed in removing the uploads of the sphere builders?