The novel takes places centuries in the future when humanity has colonized many star systems. Another race, the Duglaari or "Doogs" is slowly conquering human systems, herding the inhabitants into barren areas where they simply starve to death. The two races have approximately equal technologies, and space battles are decided by superior numbers, with the Doogs always having the advantage.
The colonists are awaiting the emergence of saviors from "Fortress Sol", the Solar System, which has been closed off to all ships since the early days of the war. Before sealing themselves off with billions of space mines and robot ships, the Solarians, as they are known to the colonists, promised to re-emerge with an answer to the numeric superiority of the Duglaari.
Returning from yet another lost system, a fleet commander called Palmer finds that a group of Solarians has contacted his superiors and wishes him to accompany them on a mission. The mission is to journey to the Duglaari home world and end the war.
Along the way, he discovers that they are different from any people he has ever known. He was raised in a hierarchical military society, where computers make all important decisions, including the conduct of battles. He is amazed that the Solarians use computers very little, relying on their innate skills to pilot spaceships, navigate, and decide on tactics. They rely on the "Organic Group", the idea that humans have individual talents allowing them naturally to adopt roles in small cohesive groups. One man, stereotypically handsome and charismatic, is Leader. Another takes the role of Gamesmaster, intuitively understanding probability and psychology. The group includes a pair of telepaths, and a mysterious woman who has no specific role, except that the Group is better with her than without. Her role is described as "Glue". Part of her job is to relax their guest and prepare him for his role, which involves offering sexual as well as spiritual comforts.
The Solarians take Palmer to Fortress Sol, behind all its defenses, and tour the system so he can see Earth for the last time. The Duglaari fleet arrives and penetrates the defences, unleashing massive bombardments that destroy all the habitable planets. When all seems lost, a stardrive in a ship on Mercury is activated, and Sol explodes, annihilating the Doog fleet. Suddenly the Solarian scheme becomes clear to Palmer. By goading the Doogs into risking a large part of the fleet, they have destroyed so many ships that from now on, the colonists will always have the advantage. Palmer's humiliation was a necessary part of the deception. He believes that Earth sacrificed itself to save the colonies. He is wrong in one detail, however.
In the final chapter, it is revealed that humanity had evacuated the Solar System and is traveling between the stars in massive Space Arks. Faster than light ships cannot be larger than a certain size, but the Arks, proceeding slower than light, can be as big as necessary. In a matter of a few decades, they will reach Alpha Centauri and humanity will reunite to defeat the Duglaari. In addition, with Earth destroyed they will cease looking back to the home world and will conquer the galaxy.