The old man, whose name is Santiago, is a fisherman who lives alone near Havana. Incredibly poor, he sleeps in a shack and sets out each day on a small skiff to try to catch himself some fish to eat or sell. For awhile, the boy, named Manolin, accompanied the old man each day, to learn from him and assist him. Unfortunately, the old man went weeks without catching anything, so the boy's parents made him stop accompanying the old man. When the story begins, the boy is probably around twelve years old, and the old man has gone eighty-four days without catching a fish. The boy comes to his shack to talk to the old man about baseball, as they both greatly admire Joe DiMaggio, and supplies the old man might need. The boy likes to bring the old man whatever he can, such as food, bait, and clothing. He also helps the old man carry his heavy mast and sail out to his boat each morning. They drink a cup of coffee together before the old man heads out on his skiff alone.

     On this day, the old man decides to venture out farther than he normally would. He sees a bird, which leads him to some flying fish. He is able to catch a tuna, which he eats before he notices his deep line has something big on it. Gingerly, he allows the line out in order to let the fish swallow the hook more deeply before he tugs to secure it. From the way the fish pulls the entire boat, the old man knows he has a very large catch. While waiting to reel it in, the old man uses a smaller line to snag a dolphin, which contains two flying fish inside of it. He eats much of these fish to keep up his strength as reeling in the big fish takes him more than two days. He fights the need for sleep and his fatigue of constantly keeping hold of the line before the fish finally starts to circle the boat.

     When the fish, which he estimates is two feet longer than the boat, is close enough, the old man stabs him with his harpoon to kill him. Then he uses most of his rope to tie the fish to the side of the boat so that he can bring him to the shore. The old man lets the current take him, but he fears that sharks will sniff out the blood of the dying fish, which they do. The first shark takes a hefty bite before the old man stabs him with the harpoon. The next two sharks take off about a quarter of the catch before they die, taking the old man's harpoon with them. The old man fashions a new harpoon by attaching his knife to a broken oar stick. He uses this weapon on the next shark to arrive, but unfortunately, the knife breaks when he goes to remove it from the shark's skull. Later that night when a whole group of sharks arrive, Santiago must blindly swing at them with his oar and tiller, but it is no use. They eat what's left of the catch.

     The man reaches the shore and must drag his mast and sail home despite his lacerated hands and exhaustion. The boy finds him the next morning and cries for what has happened to the old man. Other fisherman measured the skeleton and found it was eighteen feet long. Even though the old man successfully caught up and could prove what he'd done, he got nothing for it.