In a future where the polar ice caps have melted and Earth is almost entirely submerged, humankind survives on ramshackle floating communities known as atolls. People adapt to this vast ocean realm, haunted by myths of a fabled "Dryland." Among the survivors is a solitary, enigmatic figure with gills and webbed feet, allowing him to live below the waves. This mariner scavenges the seafloor for soil and relics of the old world, trading them for supplies to sustain his nomadic life on the water.

The mariner's existence takes an unexpected turn when he encounters a woman and a young girl on an atoll. The girl has a mysterious tattoo that map-seekers believe will lead them to Dryland. The ruthless Deacon leads a group of raiders known as the Smokers, who attack the atoll in search of the girl for their own malevolent reasons. The mariner, despite being mistrustful of others, finds himself entangled with the woman and child, and must decide whether to continue his lone existence or help protect them from the Smokers chasing the legend they carry with them.