In a distant future, the cataclysmic melting of the polar ice caps has resulted in catastrophic sea level rises, submerging every continent. Humanity's survivors are now scattered across the vast oceans, living in isolated communities comprised of fragile artificial islands and aging ships.
Among these wanderers is the enigmatic Mariner, portrayed by Kevin Costner, who roams the seas aboard his trimaran. Upon entering a floating atoll in search of a rare and precious commodity—dirt—he faces immediate danger as the inhabitants, fearing his mutant nature with webbed feet and gills, vote to dispose of him in a toxic brine pool.
As fate would have it, chaos ensues when pirate raiders known as the Smokers invade the atoll, driven by the urgent search for an orphan girl named Enola, played by Tina Majorino, who has a vital map to the elusive Dryland tattooed on her back. Enola, along with her protector, Helen, skillfully portrayed by Jeanne Tripplehorn, hopes to escape the clutches of the Smokers with the expertise of the atoll's inventor, Gregor. However, Gregor's plan goes awry, forcing Enola and Helen to seek refuge with the Mariner, who reluctantly agrees to help them despite his initial discomfort with their presence.
Chased relentlessly by the Smoker leader, the Deacon—an antagonist played by Dennis Hopper—the trio faces numerous skirmishes. Tensions rise when Helen's misguided actions during a confrontation with the Smokers lead to damage on the Mariner's vessel, prompting a furious reaction where he cuts their hair short as a symbol of his irritation. However, as time passes, the Mariner begins to bond with Helen and Enola, even teaching Enola the essential skill of swimming.
In a daring moment, Helen's curiosity about the Mariner's workings drives them underwater to explore the submerged ruins of Denver, where he collects dirt for trading. But the danger is ever-present, as the Deacon and his Smokers launch a surprise attack, burning the Mariner's boat and abducting Enola. In a suspenseful underwater scene, the Mariner and Helen share a kiss of life while he provides her with oxygen.
After a narrow escape and regrouping with Gregor at a new atoll, the Mariner embarks on a mission to rescue Enola from the Smokers. In a dramatic standoff aboard the Exxon Valdez, the Mariner threatens to ignite the oil reserves to force the Deacon's hand. The resulting explosion allows him to break free with Enola, but not without peril, as the Deacon continues to pursue them, aiming to reclaim the map.
Ultimately, with Gregor's help deciphering the coordinates of Dryland, they navigate their way to what appears to be the pinnacle of Mount Everest, miraculously still above water. There, Enola confronts her past, discovering the remains of her parents as the group vows to restart civilization. Yet the call of the ocean pulls the Mariner away, leading him to build a new vessel and sail off into the horizon, leaving behind a world he feels he can never truly belong to.
