Set in a far future Pacific Northwest where only islands remain surrounded by a corrosive Rust Sea, The Seasteaders follows Nata, a woman with special Iron Blood, who transforms to rescue humanity from the rising oceans and war over the remaining land.
As a girl, escaping from her abusive father, Nata receives her first token from the Mulians, a race of telepathic sea creatures. She grows up on the island of La Merde where she falls in love with twin brothers, Deuc and Georgios. While Deuc, a seductive musician, schemes with a band of pirates, devoted Georgios shares Nata's vision for a peaceful world.
At university, Nata joins The Seasteaders, a group of Iron Bloods, who are planning to build a rust proof structure out on the water and create their own war-free utopia. War over the remaining land escalates on La Merde as the ocean rises.
When Nata's friends escape to their Seastead, they encounter the Mulians. The Mulians offer a way to help humanity, but to learn it some Seasteaders must go to their underwater city, Mu, and live among them. Nata and Deuc (a spy for the pirates and General Balor) travel to the depths of the ocean. As Nata lives in Mu and eats Mulian food, she begins to transform. She continues her romance with Deuc, but longs to return to Georgios.
After two years, Nata and Deuc return to the surface. Nata has completely changed and worries whether Georgios and the Seasteaders can accept her transformation.
She must convince them that to save humanity from continual war and defeat the General, they too must change. After they save the world and make these sacrifices, Nata hopes she and Georgios can be together again.
