The action begins on a Friday night in Saigon in April 1975. Backstage at Dreamland, an after-hours club which is frequented by American Marines, a group of bar-girls is preparing to perform. The Engineer, who owns the club, introduces them to a new girl, Kim, a 17-year-old orphan who has fled the countryside after seeing her family destroyed.

   Afraid that the enemy forces of the Viet Cong will kill them if they are left behind when Saigon falls, the bar-girls are eager to connect with an American G.I. who can get them out of the country. The Engineer wants to get his hands on a visa so that he can escape when the Americans pull out. He sends the girls on-stage for a sham beauty contest to select a "Miss Saigon."

   A group of Marines enters the bar singing The Heat Is On In Saigon. Among them are two friends, Chris and John. Although Chris is reluctant, John insists that a night at the club is what they need to escape the increasing sense of danger from the approaching enemy, the Viet Cong.

   Chris is astounded by Kim's innocence when the Engineer pushes her forward to solicit the crowd. While the other girls offer provocative invitations, she speaks of a "heart like the sea" and "a million dreams are in me." Gigi, one of the experienced bar-girls, is chosen "Miss Saigon." She is "won" by a Marine and begins to dance with him. The scene freezes as Gigi sings The Movie In My Mind, describing the dream of a new life in America she envisions each time she is in the arms of a G.I. Kim sings of the movie in her mind - a dream of a man who will not kill and will keep her safe.

   Sensing that Chris is interested in Kim, John bargains with the Engineer to secure her for his friend. Kim and Chris dance as a Marine plays the saxophone. Chris takes Kim to a tiny room overlooking the moonlit city.

   Chris, deeply touched by what he has shared with Kim, quietly leaves the room while she is sleeping. Walking through the street, he sings Why God Why? questioning why he has had this profound experience when he is about to leave Saigon. He is drawn back to the room and tries to give Kim money. Kim refuses to take it and tells him that she has seen her parents die in flames. She was betrothed to a man she didn't love before her village was destroyed. Now she has no future except to sell herself at Dreamland.

   Chris asks her to live with him and they express their love for each other (Sun and Moon). As they part, she says that she will bring friends to bless their room with paper unicorns and perfume in keeping with her traditions.

   Chris calls John at the Embassy, where they both work, to tell him that he needs time to spend with Kim. John says that Saigon is falling apart and urges him to come to the Embassy right away. Chris bargains with the Engineer for another night with Kim.

   Gigi and the other bar-girls are helping Kim move in with Chris. They set up a small shrine with photos of Kim's parents. Kim leads Dju Vui Vai, a song which she tells Chris is traditionally sung at weddings.

   In the midst of their celebration, the door flies open and Thuy, Kim's fiancee, appears. He has come to claim Kim and save her from the shame of being a bar-girl. When he sees Chris and the shrine to her parents, he is outraged. Kim tells him that her parents' promises died with them. Kim refuses to go with Thuy. He leaves, saying that she is his until she dies. He predicts that "Saigon will fall and so will your G.I."

   Chris tells Kim that he has decided to bring her with him to America. They sing The Last Night of the World, recalling the saxophone that was played at their first meeting. Its lonely sound is now a "cry that tells us love goes on and on." Kim and Chris promise each other that their "lives will change when tomorrow comes."

   Three years have passed during which the Viet Cong have taken over Saigon (The Morning of the Dragon). The Engineer is brought in by two soldiers. We learn that he was taken prisoner by the Viet Cong and has been forced to work in rice fields. Although his captors attempted to brainwash him, they have been unsuccessful. He is led before Thuy, who is now a Commissar. Thuy demands the Engineer's assistance in finding Kim and gives him 48 hours to accomplish this deed.

   Kim is living in a small room which she shares with a group of Vietnamese. She sings I Still Believe, remembering her nights with Chris. In America, at the same time, Ellen, who is now married to Chris, sits beside her sleeping husband on their bed, singing the same song. Ellen knows that Chris is tormented by secrets that he cannot share with her, but she vows to be his wife for life. A world away, Kim, alone in a room full of strangers, makes the same vow.

   The Engineer appears with Thuy. Thuy wants to forget Kim's transgression and marry her as their parents decreed. She says she is still bound to Chris. Thuy calls her a fool and summons his men into the room, where they threaten Kim and the Engineer. Thuy again demands that Kim agree to his wishes. The Engineer is sent out of the room and Kim reveals her secret to Thuy: her two-year-old son, Tam. Thuy says she can't keep the child since he intends to marry her. He calls the child his enemy and draws out his knife to kill the little boy. Kim pulls out a gun and threatens to shoot him. He will not relent. She kills him.

   Kim is horrified by her action, but decides that she and Tam must find a way to reach Chris in America. The Engineer is making his way through the city (If You Want To Die In Bed). He finds his way to the remains of Dreamland and opens a trap door. There he finds a box of counterfeit watches he had hidden. Planning to sell them in Bangkok and then travel to the United States, he is about to leave when Kim appears. She begs for his help, and he realizes that the baby is their passport to America. He leaves to buy their passage on a boat to Bangkok. Kim consoles Tam and promises him that she will risk anything for him (I'd Give My Life For You). The Engineer, Kim, and Tam join a group of Vietnamese boat people who sing "No place, no home. No life, no hope. No chance, no change. No regret, no return, no good-bye."

   Act II

   John now works on behalf of Bui-Doi children, the children of American and Vietnamese conceived during the war. He is showing a film relating the plight of these children at a conference in Atlanta (Bui-Doi). The images in the film of the small forgotten victims reach out to haunt the veterans of the war, including Chris, who has been summoned to the conference by John.

   Chris learns that Kim has escaped to Bangkok and that he is the father of her two-year-old son. Chris is distraught. Believing that he would never see Kim again, he has married Ellen. John suggests that Chris tell Ellen the truth. Then they can go to Bangkok together to face the situation.

   In Bangkok, the Engineer has landed a job as the doorman and barker at a sleazy nightclub called "Le Moulin Rouge" (What a Waste). John arrives, and the Engineer leads him to Kim. John tries to tell her the truth about Chris, but she insists on showing off her son. John is torn between telling her the truth himself and allowing Chris to tell Kim about his marriage (Please). He tells her that Chris is in Bangkok and that he will come to see her. When John leaves, the Engineer tells Kim not to wait for Chris to come to her but to go to his hotel at once. She asks her parents' pictures for their blessing, and the Engineer leaves to find out where Chris is staying.

   As she waits for the Engineer, Kim falls asleep. Thuy's ghost appears to haunt her. He asks her where Chris was the night Saigon fell. We travel back to the past she remembers. After getting her a visa so that she could return to the United States with him, Chris left her in their room with a gun and went to work at the Embassy. He assures her that they will have plenty of warning when it is time to escape. However, when he reaches the Embassy it is a wild scene, with mobs of Vietnamese pounding at the gates, begging to be evacuated. Kim comes to the gate just as the order is issued that no more Vietnamese can enter the grounds. Although Chris and Kim struggle to reach each other, their efforts are in vain (The Fall of Saigon). He is forced to board the last helicopter to leave Saigon. She is left crying at the gate, singing "My heart, against all odds, holds still" as their love affair is swallowed up and the old city disappears forever.

   Kim wakes up and sings of her unwavering love for Chris. The Engineer brings the address of the hotel, and Kim runs through the city to find him. At the same time, John is bringing Chris through the streets to find her.

   At the hotel, Ellen mistakes Kim for a maid. Kim assumes that Ellen is John's wife. Ellen, realizing who Kim must be, tells her that she is Chris' wife. Kim is stunned. As the truth sinks in, she insists that Ellen and Chris must take Tam back to America with them. Ellen refuses, saying that Tam belongs with Kim. Kim says that Ellen and Chris cannot condemn her child to be part of the dust of life.

   Ellen is very upset at having met Kim and feels that part of Chris will always love Kim. Chris and John return, and Ellen confronts Chris with her doubts about his love for her (Now That I've Seen Her). He explains that his relationship with Kim happened when he was a different and deeply confused man. Kim put him in touch with his feelings at a very complex time. But only with Ellen did his life begin again. They reaffirm their love for each other.

   John reminds them that they must resolve Tam's future. Chris says that Kim and Tam must remain in Bangkok with his financial support because Ellen could not accept the idea of having Kim in the United States. John knows that Kim will not accept the idea of having Tam stay in Bangkok and warns them to reconsider (The Confrontation).

   Outside, Kim is promising Tam that he will have a new life in America. She knows that Chris will come for his son. She goes back to the club and lies to the Engineer, saying that Chris is coming for them that night. She says that the Engineer must pack his things and prepare for the journey to America. The Engineer sings about The American Dream and shares his vision of America.

   Kim dresses Tam in his best clothes. She tells him that his father is coming to take him home. She kisses Tam and hears Chris and the Engineer approaching. She gives Tam a toy and sends him out to join the Engineer. She takes Chris' gun out of her bedside table and disappears behind the bed curtain. A gunshot is heard. Chris runs into the room. Kim's body falls out from behind the curtain, and Chris cradles her in his arms as she dies, singing "How in the light of one night have we come so far?"