In 2500, as a result of the sea levels rising over 7,600 metres (24,900 ft), every continent on Earth is now underwater. The remains of human civilization live on rugged, floating communities known as atolls, having long forgotten about living on land. It is believed that a mythological “Dryland” exists somewhere in the endless ocean, though some refuse to believe that it does, calling it a myth.

The Mariner, a lone drifter, arrives at an atoll on his trimaran to trade dirt, a rare commodity, for other supplies. When the atoll’s residents see that the Mariner is a mutant, with gills and webbed feet, several accost him and he kills one in self-defense. As a result, the Mariner is sentenced to be “recycled” by being drowned in a pit of organic sludge. Before this can happen, however, the atoll is attacked by the Smokers, a gang of pirates seeking a young girl named Enola. According to their leader, the Deacon, she has a map to Dryland tattooed on her back. Enola’s guardian, Helen, attempts to escape with her on a gas balloon dirigible created by inventor Gregor, but he accidentally releases it with only himself onboard.

Helen quickly frees the Mariner, on the condition he take both of them with him. The three escape to the open sea aboard the trimaran, escaping the Smokers. Later, when the Smokers send a seaplane to find the Mariner’s boat, Helen shoots it with a harpoon. Its cable causes damage to the boat before being severed, causing the Mariner to cut Helen’s hair (and later Enola’s) as punishment. During their quest, the trio encounter another drifter who is killed by the Mariner after a trade-gone-wrong, as well as a trap set by the Smokers, a mutated shark the Mariner kills for food, and drawings Enola made on the boat with crayons belonging to the Mariner, angering him.

Later, Helen explains that she believes humans once lived on land and demands to know where the Mariner collected his dirt. He takes her in a homemade diving bell to show her the underwater remains of Denver, Colorado and the soil on the ocean’s floor, seeming to disprove Helen’s belief. When they surface, they find that the Smokers have caught up, threatening to kill them if they do not hand over Enola, who is hiding aboard the boat.

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