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.
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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, they decide to “recycle” him by drowning him in a pit of organic sludge. Suddenly, the atoll is attacked by the Smokers, a gang of pirates seeking a girl named Enola.
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According to their leader, the Deacon, Enola has a map to Dryland tattooed on her back. Enola’s guardian Helen attempts to escape with Enola on a gas balloon dirigible created by Gregor,
an inventor, but the balloon is released early by mistake. Helen quickly frees the Mariner and insists that he take the two of them with him.
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The three escape to the open sea aboard the trimaran, pursued by the Smokers. Helen’s escape results in damage to the Mariner’s boat, and he angrily refuses to take her to Dryland. He then cuts her
hair and then Enola’s and decides to take them anyway. During their quest to find Dryland, many other events happen to the crew, such as a drifter approaching them and being killed by the Mariner after a trade,
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coming across a trap by the Smokers, finding a large mutated shark, and discovering Enola’s drawings of various Dryland objects that the Mariner recognizes from National Geographic magazines.
Later, Helen explains that she believes humans once lived on land and demands to know where the Mariner collected his dirt. He provides her with a homemade diving bell and takes her to
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view 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 to them,
threatening to kill them if they do not hand over Enola, who is hiding aboard the boat. The Smokers abduct Enola and try to kill Helen and the Mariner. The Mariner takes Helen, and they dive underwater to avoid capture,
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with the Mariner’s gills helping Helen breathe. When they surface, they find that his boat has been destroyed. Gregor manages to find them and he takes them to a new
makeshift atoll inhabited by the survivors of the first attack using his gas balloon dirigible.
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