Santiago, an aging and experienced fisherman, has gone 84 days without catching a fish. His young apprentice, Manolin, has been forbidden by his parents to fish with him due to his bad luck. Despite his streak of misfortune, Santiago remains hopeful and determined. He sets out alone into the Gulf Stream, far from the shore, determined to make a great catch and prove himself once more.

On the 85th day, Santiago hooks a massive marlin, one of the largest he has ever encountered. The fish is so powerful that it drags his small skiff deep into the sea. Santiago, using all his skill and strength, holds onto the line, enduring the fish’s struggle for two days and nights. As he battles exhaustion, hunger, and pain, he admires the fish's strength and dignity, feeling a deep sense of connection with his adversary. Throughout the struggle, Santiago reflects on his past, his love for baseball, and his admiration for Joe DiMaggio, drawing strength from these thoughts.

On the third day, the marlin finally succumbs to exhaustion, and Santiago, triumphant but weary, lashes the giant fish to the side of his boat. However, his victory is short-lived as sharks, attracted by the marlin’s blood, begin to attack. Santiago fights them off as best as he can, using a harpoon, a knife, and even a club, but they devour the marlin, leaving only its skeleton. Defeated yet unbroken, Santiago returns to shore, where the villagers, including Manolin, are awed by the size of the marlin’s remains. Manolin, deeply moved by Santiago’s ordeal, vows to fish with him again.

The novel ends with Santiago falling into an exhausted sleep, dreaming of his youth and the lions he once saw on African beaches, symbolizing his undying spirit and longing for greatness. Despite his physical defeat, his resilience and the respect he has earned affirm his unyielding strength, making his journey a testament to the human condition.