Because turbulent weather has prevented them from fishing in the sea for seven consecutive days, the inhabitants of a west Mexican coastal village have become anxious and hungry. After one pearl diver, Kino, and his wife Juana eat the last of their beans, their baby Juanito is stung by a scorpion. The couple rushes Juanito to the nearest doctor, but he refuses to see them because they have no money. Despite the lack of medical attention, Juanito survives the scorpion's poison, and Kino and Juana are finally able to take their boat out to sea. When Kino accidentally drops his knife while under water, he discovers a deep bed of untouched oysters. Determined to obtain one particularly large, old oyster, Kino risks his life diving in the deep water. His efforts are rewarded when the oyster yields an enormous, perfectly rounded pearl. News of Kino's good fortune spreads quickly through the village, and the couple is treated with awe and respect by their neighbors. When asked about his plans for the pearl, Kino replies that he will buy a rifle for himself, clothes for his devoted wife and a good education for Juanito. "The pearl will make us free," he says. That night, a festival is held in honor of Kino's pearl, and the couple is accosted by the village doctor, who now insists on treating Juanito. After Kino rejects the greedy doctor's ministrations, he is warned that Gachupin and Sapo, two men who have ingratiated themselves with the couple, are going to steal the pearl. Later, in the cantina, Kino drinks and fights with his new "friends," who then pay a barmaid to distract the fisherman while they steal his pouch. When Gachupin and Sapo discover that the pouch is empty, they try to harass Kino into revealing the pearl's whereabouts. Kino misguides them long enough to return home and is relieved to learn that the wise Juana took the pearl herself. The next day, Kino and Juana leave to sell the pearl to the village patron. Although unaware that it was the patron who ordered Gachupin and Sapo to steal the pearl, Kino and Juana are reluctant to trust him when he offers 900 pesos for their gem. When all of the pearl buyers then pretend that the pearl is nearly worthless, Kino and Juana depart for home. That night, Kino fends off an attacker, and his trusted neighbor, Juanito's godfather, advises him to leave the village for good. Distraught, Juana begs Kino to destroy the pearl, which she believes will bring them death and loneliness, but Kino insists that the gem will buy a better future for Juanito. After Kino falls asleep that night, Juana tries to throw the pearl into the ocean, but Kino wakes in time to stop her. Gachupin and Sapo then attack Kino on the beach, and he is forced to kill both of them. Now desperate, Kino and Juana decide to leave the village with Juanito, but their small boat quickly capsizes in the still-rough water. The couple runs to the swamp and are pursued by both the patron and the doctor. The patron shoots and kills the doctor just as he is about to shoot Kino, but is unable to capture the fisherman. After the couple escapes the swamp, they see the patron and his men heading in one direction and flee in the opposite direction. That night, however, an exhausted Juana discovers that her cut bare feet are leaving a trail of blood and begs Kino to go on without her. Kino refuses to abandon his wife and makes a crude pair of shoes for her. Eventually the family reaches the hut of a sheepherder, who gives them water and is later killed by the patron for his kindness. The family's desperate flight ends when they become trapped on an ocean bluff. Seizing his opportunity, the patron opens fire on the couple and hits Juanito. Enraged, Kino then kills the patron, but loses Juanito. Like "sleepwalkers," the now childless couple returns to the village and heads for the ocean. After Kino tosses the pearl back to the sea, he and Juana clutch each other hands.