In the future, humans have formed an intergalactic empire ruled by aristocrats. During a time of war with the Plith, an empire of ant-like alien bug people, ambassador John Craig, a formerly Liberal Earth man in his 30s, is dispatched to the strategically important planet Kossar, a human colony that was settled by the Carlyle Society as a place of exile for political extremists and now is ruled by an oligarchical high council of seven nobles, each of whom is in charge of a different domain with its own traditions. Their boredom and absolute power have driven them to madness, to the point that Kossar's entry into the empire has been stymied by the Man-Inhabited Planets Treaty's clause (written by Craig) against alliances with slave owning societies, due to its practice of kidnapping humans to become illegal playthings of the galaxy's super-rich.

Craig, who now is campaigning to bring Kossar into the empire, had previously been to the planet when the passenger ship on which he was travelling on a return trip from the Betelgeuse Conference was captured by space pirates. While en route to Kossar, one of the pirates awakened Craig and the other prisoners to rape a 15-year-old virginal redheaded female captive in front of them; the rapist's fellow pirates later hear of this and dock his pay as punishment for spoiling her market value. Craig then spent two years as a slave of the beautiful, sensual, and sadistic Lady Morgan Sidney, the only female member of the oligarchy, with whom he became romantically involved. Together, they lived in her castle, ruling over and engaging in sexual relations with those under their dominion, including an enslaved teenager at a clinic used to breed enslaved people. When Craig stumbles on hints of an alien invasion, he realizes he must escape to save humanity.

Craig is depicted as undisturbed by Lady Morgan's sadism. When he is ordered to sexually assault the enslaved teenager, he enjoys his participation in the act.

Space Relations saw increased public attention after Jeffrey Epstein, Barr's former employee, who he hired without teaching credential at a high school, died in jail, due to the similarities between the violent sexual depictions in Space Relations and Epstein's sex trafficking activities and obsessions, along with the crimes of his associate Ghislaine Maxwell. Sellers of the novel on eBay explicitly advertise its connection to Epstein in their descriptions of it. It was also included as a key plot point in the season 4 finale of the legal drama The Good Fight.