The movie Waterworld is a post-apocalyptic story about a mutant named Mariner (Kevin Costner). As the polar ice-caps in the world have all melted, very little land remains, causing some humans to mutate into growing webbed feet and gills, to adapt to their new environment.
'Normal humans' however, hate, shun and attempt to kill what they consider to be unnatural creatures. Mariner (Costner) is captured by a floating village, in which dirt is more precious then gold, and orange trees are worth an arm and a leg.
During his execution, locked in a cage, the settlement is attacked by 'smokers', the villains, saving Mariner (Costner). He makes a lucky escape, reluctantly saving Helen (Jeanne Tripplehorn). Mariner then proceeds to find a legendary piece of land (Mt. Everest), which is still dry and habitable by land-living humans.
Defeating the 'Smokers' on the way, Mariner, Helen, and Helen's adopted daughter manage to finally reach this paradise.
A creature of the sea, Mariner finds himself unable to tolerate living on dry land, and sails off into the distance.
An undetermined amount of time in the future the polar ice caps have completely melted and the surface of the Earth is an endless ocean. A narrator tells us that the survivors of the ancient deluge "have adapted to a new world."
One such survivor, known only as The Mariner, lives on a large sailed boat, a "tri-maran", and lives by recycling his own urine through a special filter. He drags the ocean floor for artifacts to sell at atolls, man-made island habitats where most of the last humans live. While scavenging under the ocean surface, someone boards the Mariner's boat and steals the fruit from his tiny lime tree. The Mariner surfaces and sees the other pilot's boat. He holds him at bay for a few minutes with a harpoon gun, telling the pilot he's seen the boat before but doesn't recognize the man. The man says he took it fair from the owner when he found him dead. The pilot tells the Mariner about an atoll where he can find supplies.
Just then, the two pilots spot two jet skis. Aboard both are Smokers, pirates who thrive on terrorizing the seagoing inhabitants of the Earth, smoke cigarettes and use refined oil for fuel. When they spot the pouch of recovered items the Mariner found on the ocean floor, they race for it. As they do, the other pilot shows the Mariner that he stole his limes. The Mariner throws several switches on his boat, raising a giant sail, giving his boat great speed. He recovers his bag of goods, then steers his vessel toward the pilot who ripped him off. He breaks the man's mast, leaving him helpless against the Smokers, who cease their chase of the Mariner when they catch up with the other pilot. The Mariner sails off.
After several days the Mariner arrives at the huge atoll the other pilot spoke of. At first he's refused admittance, but the atoll's guards allow him in when he shows them he has a jar full of dirt, the world's most precious substance. Inside the gates, he finds that the people live in poverty and are near starvation. One small group is conducting a funeral where the deceased is "recycled" & dropped into a pool of viscous waste. The Mariner docks his boat & is met by the atoll's security officer who tells him not to make any trouble & that he has no more than two hours to conduct his business. He first talks to an accountant of the colony who offers him a large amount of currency ("chits") for the dirt he was carrying. The Mariner demands twice the amount and gets it. At the atoll's store/bar, a shady man named Nord is asking another man, desperate for a drink of fresh water ("hydro"), about a girl with a tattoo. Supposedly, the tattoo has markings that give the location of "Dryland", a piece of land not submerged in the deluge that covered the Earth. Nord spots the girl, Enola, who is in the care of the bar's owner, Helen. Helen quickly hides Enola, but not before Nord catches a glance of the tattoo on the child's back.
The Mariner finds the shop and spends his money on hydro, a tomato plant, & buys Helen's shelves. Nord approaches the Mariner, asking him about his new found wealth, but the Mariner remains unfriendly. He marches back to his boat and begins to load his purchases on it. A small group of people approach him, asking if he'll have sex with a young girl to impregnate her. When he refuses, the atoll's council of elders become suspicious and decide to detain him. When he's grabbed by two men, they discover he's a mutant, with webbed feet and gills behind his ears. Declaring him abominable, they begin to beat him. The Mariner fights with a few guards, plunging into the lagoon with one of them, killing him when the man draws a knife. He's hauled out of the water and beaten further until the Enforcer stops them, saying its not their way to dispense this type of judgement.
Later that night, while in a cage, the Mariner spots Nord leaving the atoll unannounced. He's visited by Helen & Enola's friend, Gregor, the atoll's scientist, who wants to study him - the Mariner remains unfriendly. Helen visits him as well, asking if he'll take her & Enola if he leaves. He asks her to give him a dock cleat to pick the lock on his cage. Helen refuses.
The next day the council has decided that the Mariner is a danger to them and he's to be dropped in the recycling muck and drowned. Just then, an armada of Smokers, led by the Deacon, launches a huge attack on the atoll. The compound dwellers are outnumbered and out-armed. During the battle, the Mariner's cage is dropped into the muck when a Smoker launched over the wall collides with it. As he sinks, drowning, Helen tells him she'll free him if he takes herself & Enola with him. She opens his cage and they both work together to get Enola onto the boat and allow it to escape the compound. They barely manage to get the gates open and flee. The Deacon spots them and orders his fleet to attack his boat. The Mariner fires a harpoon at the armada's primary gunship, using a cable to turn its guns on the Deacon's boat which is loaded with gasoline. The boat explodes throwing the Deacon off. The Mariner, Enola & Helen escape.
Back at the wrecked atoll, the Deacon, now wearing a bandanna to cover a lost eye, interrogates one of the dwellers and finds out that Enola escaped aboard the Mariner's boat. The Deacon assigns his fleet a new mission to find Enola.
Aboard the Mariner's boat, the Mariner tells Helen that she can stay but Enola should be thrown overboard. Helen offers her body to the Mariner but he declines. When she picks up a harpoon gun and tries to hold him hostage, he drops his sail on her & hits her underneath it with an oar, knocking her out.
Aboard the Deacon's vessel, the Deacon finds out that they are rapidly running out of oil for refining. The Deacon resolves to find Enola and their way to Dryland, which is a matter of survival. He orders two of his crew to take the small plane they have aboard their vessel & scout the area for the Mariner's boat. While searching, the pilot & his comrade find the Mariner's boat. While the Mariner prepares a counterattack so the pilot can't give their location away, Helen shoots the plane with a mounted harpoon gun. The plane begins to circle the Mariner's boat, entwining the harpoon's cable in the sail rigging, threatening to wreck the vessel. The Mariner climbs his mast and tries to shoot the plane down, but is thrown into the ocean when the pilot is able to shoot & sever the cable. The Mariner, furious at Helen's incompetence, viciously cuts her hair short. When Enola protests, he does the same to her.
Some time later, the Mariner spots another sailor. Helen loudly says that they may be able to trade with the man for food and they stop to talk to him. The man turns out to be somewhat paranoid and neurotic and, at first, wants to trade some sheets of paper for sex with Enola. Helen & the Mariner refuse & Helen agrees to have sex with the man belowdecks. The Mariner reads the sheets of paper, which are pages from National Geographic magazine & are strikingly similar to pictures that Enola had been drawing, indicating that she's been on Dryland. The Mariner negates the deal with the drifter for sex, rescuing Helen. When the man tries to fight his host, the Mariner kills him and salvages items from his boat.
Helen still insists that she & Enola need food. The Mariner preps a harpoon gun and jumps off the back of the boat, using himself as bait. A large sea creature swallows him live and he kills it with a couple of blasts from his gun. He feeds Helen and Enola the flesh from the giant creature. While he cooks extra meat for storage, Enola notices his webbed feet, saying she'd like to have feet like his. When Enola suggests that the Mariner isn't talkative & a bitter loner, he tells her she doesn't listen enough to the world around her.
After a few days Enola and the Mariner come to an understanding that leads to a friendship. He teaches Enola to swim, something she'd never been trained to do.
A few more days after, the three come to an outpost. The Mariner tries to communicate with them but the inhabitants don't respond verbally and simply keep waving. The Mariner uses an underwater periscope and sees Smokers hiding below the outpost, which had recently been pillaged by the Deacon and his crew. The Mariner raises his sails and sets off, barely escaping the Smokers and their high speed vehicles. The Deacon takes aim with a rifle and hits the Mariner in his side, wounding him.The Deacon decides to let them escape, believing it won't be very long before they catch up to them.
Helen grows impatient, wanting to know when they'll find Dryland. The objects on the Mariner's boat and the dirt he'd sold at the atoll convince her that he can take them directly there. The Mariner says he's sailed to just about every point on the globe and has never found it, however, he will show her where he got the objects and the dirt. He fashions a crude diving bell and takes Helen into the depths of the ocean, showing her a long-sunken city (Denver, Colorado) where there is plenty of dirt and items like the ones he's found. When they surface Helen mentions that she never wanted to believe the legends she'd always heard that the world was covered with water in a cataclysm rather than being created that way.
Suddenly the Deacon and his crew appear. Enola is already hiding and Helen and the Mariner are used as bait to flush her out. Enola is taken and the Mariner and Helen jump overboard. The Mariner kisses Helen, using his gills to breath for them both until their attackers have left. Helen and the Mariner stay on the flotsam of his boat until they're found by Gregor, the scientist from the atoll, who'd escaped in a crude hot air ballooned-vehicle. He takes them to a group that had also escaped the atoll attack and have joined their boats together in a makeshift atoll. Helen and the Mariner try to convince everyone that the world was not created in a flood but covered by it, a blasphemous concept. They all agree that they must find Enola and use the markings on her back to find Dryland. The Mariner agrees to find the Deacon's vessel and rescue the girl.
The Mariner arrives at the Deacon's boat, an ancient tanker. The Deacon has stirred his crew into a frenzy with a rousing speech about finding the path to Dryland. The crew begins to row frantically with giant oars, not knowing that the Deacon hasn't yet deciphered the markings on Enola's back. He holds her prisoner in the bowels of the ship, threatening to kill her. Enola counters, saying the Mariner will come to save her.
The Mariner is able to infiltrate the ship and find the topside deck. He approaches the Deacon's perch on the bridge and demands to have Enola back. The Deacon believes he's bluffing, until the Mariner produces a flare, holding over one of the vents leading to the tanker's oil supply. When the Deacon mocks him, the Mariner drops the torch, igniting the remainder of the oil, causing the ship to burn. The Deacon grabs Enola and tries to escape in his plane, but the Mariner stops him with a old anchor and cable, downing the plane. The Deacon jumps off his boat as it sinks, finding an old Jet Ski. Just as the boat is about to sink, Enola & the Mariner are rescued by Gregor and Helen in Gregor's flying ship. As they watch the ship sink from the air, they see the name "Exxon Valdez" on the stern.
The Deacon uses a pistol to hit the small craft, throwing Enola off and into the water. He sees that three of his crew are nearby on Jet Skis and they race toward Enola. The Mariner ties a long bungee cord to his ankle and jumps down, grabbing Enola just before the Deacon and his crew collide and their Jet Skis explode.
The Mariner and Enola are pulled up. Gregor looks over the tattoo on Enola's back and determines the direction they need to follow to Dryland. Everyone falls asleep and the craft drifts through the air for a few days. When it seems hopeless that Dryland will be found, the Mariner sees a seagull on the the craft. They drift through some clouds and see a mountainous island. On the shore, they find a tropical landscape with fresh water and food. The Enforcer finds a small hut in the woods with a couple of skeletons. Enola recognizes the home as her own. The Enforcer suggests that they bury the dead inhabitants, Enola's parents, because it was their tradition.
The Mariner decides to leave, being more comfortable on the ocean. He builds a smaller boat than his trimaran and says a tearful farewell to Enola. He promises to find more atoll dwellers and drifters and bring them back. In the extended version of the film, Enola and Helen watch him sail off from the island's mountaintop where there is a plaque saying that they're on the summit of Mt. Everest.