1904, Japan. After a brief prelude, Pinkerton, a lieutenant in the United States Navy, and Goro, a Japanese matchmaker, enter. Goro shows Pinkerton a house that he has picked out for him in Nagasaki for him to live in with his soon-to-be wife, simply known as "Butterfly." They sing "E soffitto e pareti," which translates to "And ceiling and walls." Pinkerton playfully doubts the structural soundness of the house, but Goro insists that it is very sturdy.
Goro shows him the house, which has a sliding screen, a garden, a butler, a cook, and a maid for his bride-to-be named Suzuki, who notes that Pinkerton is smiling at her. Goro dismisses each of the servants and tells Pinkerton that his wife will arrive imminently. He also tells him that the marriage registrar, the American consul, whose name is Sharpless, and the bride's relatives (other than her uncle) will also come. Butterfly's uncle, a priest, does not approve of the marriage.
Sharpless enters, having walked to the house from the city, and Pinkerton tells Goro to get them something to drink. He marvels at the view of the harbor in Nagasaki, and Pinkerton tells him that he has purchased the house for 999 years, but can break the contract whenever he wants. They talk about the fact that the Japanese law is quite loose.
Pinkerton sings an aria, "Dovunque al mondo" ("Throughout the world") about how American men must travel the world and love every beautiful woman they can find. He suggests that he is staying true to Japanese tradition in that he is marrying for 999 years, but knows that he could cancel it any month he chooses. "Life's not worth living unless he can pick the flowers on every shore," Pinkerton sings. Sharpless thinks that Pinkerton is being irresponsible, but both of the men share a strong sense of American patriotism. The aria even begins and ends with the opening flourish of "The Star-Spangled Banner." When Sharpless asks if Butterfly is beautiful, Pinkerton tells him that he purchased her for only 100 yen.
Pinkerton asks Goro to bring Butterfly to the house, as Sharpless asks the lieutenant if he really loves Butterfly. Pinkerton sings "Amore o grillo," about the fact that he cannot distinguish whether his feelings towards Butterfly are actual love, or just enchantment with her beauty and youth. He compares her to a butterfly, in that she is delicate and beautiful, and suggests that her delicacy makes him want to chase her. Sharpless urges Pinkerton not to "pluck her wings," but Pinkerton assures him that he cannot do much harm. Sharpless has some whisky and makes a toast to Pinkerton's American family, as we learn that Pinkerton has a woman waiting for him at home for a "real wedding."
Goro tells Pinkerton that Butterfly is coming with her friends, and she enters singing "Ancora un passo" ("One step more"), an aria about how happy she is to be marrying Pinkerton, and her excitement about her new life. "I'm the happiest girl in Japan—no, the world," she sings. She greets Pinkerton and he asks her about how it was to climb up the hill. She tells him that the waiting has been harder than the climb, and Pinkerton interrupts her in the middle of her compliments.
Butterfly tells Pinkerton that her family was once very wealthy and is originally from Nagasaki. She sings passionately about the fact that she has known wealth, but that she has had to work as a geisha since her family lost their money, but is not ashamed. When Pinkerton laughs at this, she is confused. She also tells Pinkerton that her mother is alive, but her father has died, then announces that she is 15 years old. Suddenly, Goro announces that the Grand Commissioner and the marriage registrar are arriving, as well as Butterfly's relatives. As Pinkerton sees Butterfly greeting her family members, he laughs to Sharpless about the fact that they have no idea that he intends to leave soon enough.
Butterfly gushes about how much she loves Pinkerton, and her cousin tells her that Goro the matchmaker offered Pinkerton to her first, but she passed. Butterfly's family talks about how powerful and handsome Pinkerton is, before Butterfly goes into the house with him.
Before we even meet "Butterfly," we know that her marriage to Pinkerton is somewhat unconventional. Pinkerton is an American living in Japan for a time as a lieutenant and has decided to purchase both a house as well as take a Japanese wife. He is an archetype of the American colonial adventurer, an American military man taking advantage of all that the culture he is visiting has to offer without actually integrating into the culture itself. He even suggests to Sharpless that he will not stay long in his new house or with his new wife, since contracts are so loose in Japan.
Indeed, even though Pinkerton is charming and handsome, he also proves to be quite callous and unfeeling, as he brags to Sharpless that he will not be staying with Butterfly very long, and plans to return to America to have a "real" wedding soon enough. While he is the protagonist of the opera, he is also portrayed as a man who does not quite understand the consequences of his actions, a man who believes that he should not be held responsible for toying with the life of his bride-to-be. He also creates a logic in which his marriage to a white American woman is thought of as a "real wedding," while his marriage to "Butterfly" is simply a short fling.
Indeed, it is not that Pinkerton does not realize he is toying with a woman's life, it is that he does not care. He uses the metaphor of the butterfly to describe just how he is approaching his marriage to "Butterfly," suggesting that he cannot help but play with her delicate and beautiful wings, even if he will cause injury. In this metaphor, Pinkerton becomes a kind of hunter, a ruthless predator that thinks only of entrapment, and the sense of power it will give to him, rather than the consequences it will have for his prey.
Puccini's romantic score reflects the European context in which the opera was written more than it does the Japanese setting. The score is lilting and lush, reflecting the romantic and dramatic nature of the narrative. Pinkerton sings in a high tenor while Butterfly sings in a soprano, which lends their music and characters a youthful and somewhat naive quality. The older, more mature characters, such as Sharpless, sing with deeper voices, as is customary in operatic stories. Additionally, the beauty of the music pulls us into the characters' emotional perspectives, giving us a window into their point of view, and inviting us on their psychological journey.
Butterfly is fragile in her temperament and attitude, but also, her circumstances have put her in a fragile position as well. Her family was once wealthy, but they lost their money, and so Butterfly has had to work as a geisha to support herself. Thus, we see a dangerous contrast between her priorities and Pinkerton's. While he sees their marriage as nothing more than an international fling, a bit of sexual diversity, for her it is a life-altering union that could help her ascend back to a position of prosperity in society.