Act I - On a flowering terrace above the harbor of Nagasaki, U.S. Navy Lieutenant B.F. Pinkerton inspects the house he has leased from a marriage broker, Goro, who has procured for him three servants and a geisha wife named Cio-Cio-San, also known as Butterfly.  To the American consul, Sharpless, who arrives breathless from climbing the steep hill, Pinkerton describes the carefree philosophy of a sailor roaming the world in search of pleasure ("Dovunque al mondo").  For the moment, he is enchanted with the fragile Cio-Cio-San and intends to undergo a marriage ceremony with her—a 999-year contract, but subject to monthly renewal.  When Sharpless claims that the girl may not take her vows so lightly, the lieutenant brushes aside such scruples, adding that he will one day wed a "real" American wife.  At that moment, Cio-Cio-San is heard in the distance joyously singing of her wedding ("Amiche, io son venuta al richiamo d'amor").  Entering surrounded by her friends, she tells Pinkerton how, when her family had fallen on hard times, she had to earn her living as a geisha.  Soon her relatives arrive and noisily express their opinions on the marriage and the bridegroom.  In a quiet moment, Cio-Cio-San shows her bridegroom her few earthly treasures, telling him of her intention to embrace the Christian faith.  With pomp, the Imperial Commissioner performs the wedding ceremony, after which the guests toast the happy couple.  Suddenly Cio-Cio-San's uncle, the Bonze, a Buddhist priest, bursts upon the scene, cursing the girl for renouncing the faith of her ancestors.  Pinkerton angrily orders the priest and the family to leave, comforting his new bride and drying her tears ("Bimba, non piangere").  They sing of their rapture amidst the fragrant, starry night.

Act II - Three years later, Cio-Cio-San still waits for her husband's return.  As Suzuki prays to her gods for aid, her mistress stands by the doorway with eyes fixed on the harbor.  The maid shows Cio-Cio-San how little money is left, but is told to have faith: one fine day Pinkerton's ship will appear on the horizon, returning to his waiting bride ("Un bel dì").  Sharpless comes in with a letter from the lieutenant telling of his new American bride, but before he can read it to Cio-Cio-San, Goro, who has been lurking outside, brings a suitor, the wealthy Yamadori.  The girl dismisses them both, insisting that her American husband has not deserted her.  When they are alone, Sharpless again starts to read her the letter and suggests tactfully that Pinkerton may never return.  Cio-Cio-San is stricken by this intimation, and proudly carries forth her son, telling Sharpless that as soon as Pinkerton knows of his son, he will return for them both.  She tells Sharpless that if Pinkerton does not return, rather than going back to her life as a geisha, she will kill herself ("Che tua madre").  Sharpless is deeply moved, and picking the child up, asks his name.  Butterfly responds for him saying "Today, my name is Trouble.  But on the day that my father returns, my name shall be Joy."  Sharpless vows to tell Pinkerton of his young son and leaves.  Cio-Cio-San, at the point of despair, suddenly hears a cannon shot in the harbor.  Seizing a spyglass, she determines that the ship is the Abraham Lincoln, Pinkerton's American warship!  Delirious with joy, she orders Suzuki to help her adorn the house with all the flowers from her garden ("Tutti fior!").  As night falls, to the accompaniment of an unseen humming chorus, Butterfly begins her long vigil through the night, awaiting the return of her beloved husband.  

Act III - As dawn breaks, Suzuki insists that Cio-Cio-San abandon her vigil, which she kept even though Suzuki and the child had long before succumbed to sleep.  Relenting, she hums a lullaby to her child, carrying him into another room ("Dormi amor mio").  Before long, Sharpless, Pinkerton, and Kate, his new wife, enter.  When Suzuki realizes the true identity of the woman, she collapses in despair, and out of consideration for her mistress, she begs that she be able to break the news to Cio-Cio-San.  Pinkerton is overcome with remorse and bids an anguished farewell to the scene of his former happiness ("Addio, fiorito asil").  No sooner has he gone than Cio-Cio-San comes forth, expecting to greet her husband, but only finding an American woman in the room.  She takes a moment to guess the heartbreaking truth.  Leaning on Suzuki for support, she agrees to give up her child, but with one condition—the father himself must come to get him.  Leaving, Kate agrees, and Cio-Cio-San brusquely sends Suzuki away.  She takes out the dagger with which her father killed himself, saying that she would rather die with honor than live with disgrace.  Just as she is about to kill herself, Suzuki pushes the child into the room.  Cio-Cio-San tearfully sobs a farewell to her child ("Tu, piccolo iddio!"), and giving him an American flag, sends him into the garden to play.  She then stabs herself, dying as she hears Pinkerton's voice calling her name.