The first-person narrator, commander of a tramp steamer, steers into the floating mass of seaweed and derelicts known as the Sargasso Sea, where his vessel and crew become stranded among a community of shipwreck survivors. The narrative recounts life among the Sargassons: their manners, ceremonies, jealousies, revolts, and inventive use of salvaged materials. A romantic attachment develops with a woman called Fidette as the narrator adapts to new customs, helps build boats, and faces plots of treason and internecine violence. The tale follows the community's struggles, the refitting of the Caribas, and the eventual attempt to steam out of the floating continent.