Urras and Annares are twin planets orbiting Tau Ceti, a star eleven light years from Earth. Roughly a million years ago, Urras, Earth, and a number of other planets were colonized by Hain. The Hainish abandoned their colonies, which subsequently evolved in different directions. The nation of A Io has become the most powerful on Urras. A highly class stratified society, it experiences an uprising by anarchists led by the revolutionary Laia Odo. The anarchists fail to bring about revolution, but they are permitted to colonize Annares, Urras’s barely habitable desert moon, where they are free to order their colony as they see fit. Once the colony is founded, the people of Annares and Urras have minimal contact. Eighty years later, the Hainish reestablish contact with Urras.

Seventy years after the Hainish contact Urras, an Annarean physicist named Shevek is invited to A Io to receive a coveted scientific prize and continue his work on the simultaneity principle, which his hosts believe holds the key to faster-than-light space travel. Shevek accepts the invitation, becoming the first person from either society to travel between the moon and the planet since Annares was colonized. Shevek sees himself as an emissary of reconciliation between Annares and Urras. Some of his fellow Annaresti consider him a traitor and attempt, unsuccessfully, to prevent his departure.

Shevek’s childhood has been spent in an “ambiguous utopia,” a society in which private property is virtually nonexistent and no individual has official power to coerce another. When he is two, his engineer mother abandons him to work in a distant city, and he spends most of his childhood in dormitories. The communal ethic, lack of family ties, and constant peer pressure to conform work tolerably well for the average child. However, others require a level of autonomy and individual expression that the regimented society theoretically allows but in practice discourages. These Annaresti include Shevek, whose mathematical genius is evident at an early age; Bedap, a profound social thinker; and other friends with artistic talents. Gazing up at the “moon” (Urras), the friends wonder whether what they have been told about the evil society their forebears rejected still holds true.

Despite the spartan communal existence and long hours spent performing menial labor, Shevek establishes himself, by age twenty, as a rising star in physics. He leaves the provincial community in which he grew up to work under Sabul, Annares’s leading physicist, at the National Institute of Sciences in Abbenay. There, he discovers that the institute’s scientists enjoy privileges that others on Annares do not: Shevek feels vaguely guilty, slightly tainted by the sin of being “propertarian,” when he is assigned his own sleeping room and allowed seconds on dessert at the institute cafeteria. Under Sabul, he learns Iotic to read the works of Atro, the leading Urrasti physicist, from whom Sabul has plagiarized most of his own work. At first, Shevek has free rein to correspond with Atro and produces important papers based on the collaboration. Sabul becomes increasingly jealous of Shevek, however, and he places barriers in the way of his publishing. The magnum opus of his early career, Principles of Simultaneity, sits on the shelf for four years, only to appear in an abridged version.

Their conflicts with Sabul and the difficulties they experience communicating with counterparts on Urras lead Shevek, Bedap, and their friends to open an unauthorized publishing and telecommunications network and to step up efforts to reestablish ties with Urras. These efforts, while not technically forbidden, meet with much hostility and opposition.

Annaresti customs are permissive with respect to sex. Shevek has ample opportunities for sexual encounters as a teenager, but he finds casual sex unsatisfactory and has become virtually celibate when he meets Takver, a fish geneticist from his hometown. The two quickly decide they are meant for each other and that they want to be permanent partners, a status their society allows but does not support. While Takver is pregnant with their daughter, a severe prolonged drought strikes Annares. The Division of Labor ships Shevek to a remote region to do manual labor. When he returns to Abbenay, he discovers that Takver has been reassigned to a distant laboratory and his position at the institute has disappeared. For four years, he labors under conditions of extreme privation. When the couple is at last reunited, they are determined to do anything it takes to allow Shevek to realize his full potential as a physicist. It is this resolve that leads to the foundation of the publishing venture to circumvent Sabul. This venture, in turn, leads to Shevek’s invitation to travel to A Io.

Shevek arrives on Urras empty-handed. He has not yet formed a coherent theory, and he has learned not to commit preliminary versions of a theory to paper where they can be plagiarized and coopted. For his first few months on Urras, Shevek is in a state of awe at the society’s opulence. His hosts install him in a faculty residence at A Io University and carefully shield him from the poverty and oppression that are still very much a part of Urrasti life. He learns for the first time of the existence of the Hainish and their plan for a federation of worlds. Atro introduces him to the works of the Terran physicist “Ainstain,” upon whose theory of General Relativity Atro has drawn heavily, though with more originality than Sabul has exhibited.

Despite being allowed to work under seemingly optimal conditions, Shevek makes no progress. The outbreak of a war between A Io and the nation of Thu leads him to suspect that, if he were to solve the problem of simultaneous transmission of matter, A Io would use the knowledge to coerce other nations and planets rather than to cooperate with them. The attitudes of A Ioan physicists seem to support this suspicion: Atro’s judgment on social issues is clouded by his aristocratic heritage and blind patriotism, and the current head of the Physics Institute, Saio Pae, is a political manipulator.

Shevek becomes aware that there is an undercurrent of support for the revolutionary principles of Odonianism among A Io’s working classes. Their revolutionary fervor fanned by opposition to the war, the Odonians have been trying to contact Shevek, because he is a citizen of an Odonian society. His first attempt to respond ends in a drunken spree. The tension revives his scientific inspiration, but he falls ill. During his illness, Shevek is tended by his manservant Efor, who tells him what life is like for the poor. Efor advises him on how to escape from surveillance at the university and puts him in contact with the revolutionaries.

Shevek, now close to solving the simultaneity problem, agrees to address a massive demonstration of workers. While he is speaking, police helicopters descend on the crowd, machine-gunning demonstrators. Shevek and a badly injured man take refuge in a warehouse basement. While they cower there, the man dies, and Shevek puts the final touches on the theory behind a practical simultaneous communication device called an ansible. His theory cannot enable faster-than-light travel for physical objects, but it will make it possible for people on different planets to communicate instantaneously, rather than waiting long weeks or months for messages to travel between worlds.

Revolutionaries smuggle Shevek out of the capital city and deposit him at the Terran embassy in Rodarred, where he is granted asylum. There, he presents the Hainish with his plans for an ansible, as a gift. They realize the device’s profound significance to forging an interplanetary federation based on voluntary cooperation for the sake of mutual benefit.

His gift received, Shevek returns to Annares on a Hainish starship, accompanied by a Hainish individual. This person is traveling as an unofficial ambassador to learn what else the unique communitarian anarchy of Annares has to offer the fledgling Federation of Known Worlds.