At sunset before the Imperial Palace in Peking, a Mandarin reads the crowd an edict: any prince seeking to marry Princess Turandot must first answer three riddles. If he fails, he must die. The latest suitor, the Prince of Persia, is to be executed at the moon's rising; bloodthirsty citizens urge the executioner on. In the tumult, a slave girl, Liù, kneels by her aged master, Timur, who has fallen from exhaustion. Young Calaf recognizes the old man as his long-lost father, the vanquished king of Tartary. When Timur reveals that only Liù has remained faithful to him, Calaf asks why; she replies it is because once, long ago, he, Calaf, smiled at her. As the sky darkens, the mob again cries for blood but greets the moon with a sudden, fearful silence. The onlookers are further moved when the Prince of Persia passes by and calls upon the princess. Calaf also demands that she appear; as if in answer, Turandot, with a contemptuous gesture, bids the execution to proceed. As the death cry is heard from the distance, Calaf, transfixed by the beauty of the unattainable princess, strides to the gong that announces a new suitor. Turandot's three ministers, Ping, Pang, and Pong, try to discourage him. When Timur and the tearful Liù also beg him to reconsider, Calaf seeks to comfort them; but as their pleas reach new intensity, he strikes the fatal gong and calls Turandot’s name.
In a palace pavilion, Ping, Pang, and Pong lament Turandot's bloody reign, hoping that love will conquer her icy heart and peace will return. The three let their thoughts dwell on their beautiful country homes, but the noise of the populace gathering to hear Turandot question the new challenger calls them back to reality.
In front of the palace, the aged Emperor, seated on a high throne, vainly asks Calaf to reconsider. Heralded by a chorus of children, Turandot enters to describe how her beautiful ancestor, Princess Lou Ling, was brutally slain by a conquering prince. In revenge, Turandot has turned against all men and determined that none shall ever possess her. Then, facing Calaf, she poses her first question: “What is born each night and dies each dawn?” “Hope,” Calaf answers correctly. Unnerved, Turandot continues: What flickers red and warm like a flame, yet is not fire?” “Blood,” replies Calaf after a moment's pause. Visibly shaken, Turandot delivers her third riddle: “What is like ice but burns?” A tense silence prevails until Calaf triumphantly cries, “Turandot!” While the crowd voices thanks, the princess begs her father not to give her to the stranger, but to no avail. Calaf, hoping to win her love, generously offers Turandot a challenge of his own: if she can learn his name by dawn, he will forfeit his life. Turandot accepts as the crowd repeats the Emperor's praises.
In the palace gardens, Calaf hears a proclamation: on pain of death, no one in Peking shall sleep until Turandot learns the stranger's name. While the prince ponders his impending joy, Ping, Pang, and Pong try to bribe him to leave the city. As the mob threatens him to learn his name, soldiers drag in Liù and Timur; horrified, Calaf tries to convince the mob that neither knows his secret. When Turandot appears, commanding the dazed Timur to speak, Liù cries out that she alone knows the stranger's identity but will never reveal it. Though she is tortured, she remains silent. Impressed by such endurance, Turandot asks Liù’s secret. "Love," replies the girl. The princess signals the soldiers to intensify the torture. Liù snatches a dagger and kills herself. The crowd, fearful of her dead spirit, forms a funeral procession. Turandot remains alone to confront Calaf, who kisses her. Knowing emotion for the first time, Turandot weeps. The prince, now sure of his victory, reveals his identity. As the people hail the Emperor, Turandot triumphantly approaches his throne, announcing the stranger's name: “Love.” Calaf rushes to embrace her and the court hails the power of love and life.