Oro Plata Mata traces the changing fortunes of two aristocratic families in Negros during World War II. The Ojeda family is celebrating Margarita Ojeda's (Andolong) debut. In the garden, Trinidad (Gil) receives her first kiss from Miguel Lorenzo (Torre), her childhood sweetheart. Don Claudio Ojeda (Ojeda) and his fellow landowners talk about war. The youngest guests mock Miguel's refusal to join the army and brand him a Mama's Boy. The celebration is cut short by news of the Fall of Corregidor where Margarita's fiance had been assigned.Margarita is then left in tears, and her future as a comfortably wealthy housewife is in peril.
As war nears the city, the Ojedas accept the invitation extended by the Lorenzos, their old family friends, to stay with them in their provincial hacienda. Two more family friends,Jo Russell(Valdez), a wealthy, liberated mestiza married to an American,and Dona Viring,an elitist housewife whose husband has left her (Villanueva),join them in the Lorenzo Hacienda. Nena Ojeda (Lorena)Inday Lorenzo (Asensio),Jo, and Viring try to deny the realities of war by preserving their pre-war lifestyle of playing mahjong and engaging in other activities enjoyed by women of their station. Pining for her dead fianc, Margarita goes through bouts of melancholy. Miguel and Trinidad turn from naughty children into impetuous adults.
As the Japanese forces advance close enough to Hacienda Lorenzo,the families move to the Lorenzo's forest lodge after a frantic escape from Hacienda Lorenzo. A group of weary guerrillas(whose members are familiar faces; friends of Don Claudio) arrive and Jo tends to their injuries. The guerrillas leave Hermes Mercurio (Lazaro) behind.After getting a lesson in sex education from Jo, Trinidad decides to lose her virginity to Mercurio. Miguel endures more comments of the "Mama's Boy"kind when he fails to take action against a Japanese soldier who came upon the girls bathing in the river. It is Mercurio who kills the Japanese soldier. Margarita comforts Miguel, who decides to learn marksmanship from Mercurio. Meanwhile, Viring's jewelry, which she considers her best friends, is stolen by Melchor (de la Cruz), Inday's trusted foreman. He justifies his action as a reward for his services. He tries to break the other servants' loyalty, but they force Melchor to leave.
Later, Melchor, along with a band of thieves return. They systematically kill all the servants, one of them Melchor's own son. They gather the women in the living room where they subject them to all sorts of atrocities: First, Viring's fingers are chopped off with a bolo to obtain rings that she had refused to give. After being offended by a slap from Inday, he takes Inday to a room and repeatedly beats and rapes her, his noble-born former mistress. He kills his wife, who enters the room after learning that Melchor has killed his own son. Nena is forced to play a complex combination of Russian Roulette and Mahjong with the bandits.
While treating Melchor's wounds, and that of her companions, Jo notices him eying the young Trinidad. Although Melchor agrees with Jo to not take the young heiress, Trining willingly goes with the bandits anyway, despite all the crimes they have committed against her family.The bandits notice that two members of the families are missing: Miguel and Margarita. After a romantic tryst the night before that had driven Trining crazed from jealousy, the two heard the bandits' arrival and hid.
After the violent bloodbath only days before,Miguel urges the survivors to resume their mahjong games to help them cope.The women are resilient, and fall back on their old habits of idle gossipping and Mahjong.
Miguel is determined to hunt the bandits down and bring Trinidad back. He catches them in their hideout,an old hospital in the city where the robbers boast of their adventures:raping wealthy young heiresses in their homes, killing their former haciendero overlords, and robbing them of their prized possessions.His courage is replaced with bloodlust,however driving him to a killing spree. In the hospital is a woman believed by Don Claudio to be the Spirit of the Mountain, but in reality, simply an unhinged woman in a party dress subjected to sexual assaults from the bandits. Miguel kills her to cap his carnage, and also as an act of mercy.
An epilogue follows the violent climax. The combined United States & the Philippine Commonwealth troops have liberated the Philippines from Japan. A party is held in the Ojeda home to announce Margarita and Miguel's betrothal.It is revealed that Margarita's ex-fiance, Ramon, actually survived the Fall of Corregidor,but is now reduced to a pathetic invalid,dressed in an army uniform, planted on his wheelchair.He sheds a tear for his now lost love. After witnessing the atrocities of the war, Don Claudio's mind has become unstable, and add to that, he has finally fallen to the curse of senility. He could not recognize his granddaughter, or anyone for that matter. Trinidad has made a new image for herself as a pernicious but very virginal young woman. The survivors attempt to reclaim their previous lifestyle, but the war has changed the world, just as it has forever marked each of them. The film ends with a quote from Trinidad about the very subject.