Belle Poitrine (née Schlumpfert) writes her autobiography with the help of Patrick Dennis ("The Truth"). As a child from Venezuela, Illinois, she lived in Drifter's Row, the poor side of town and dreamed of living on The Bluff, the rich part of town ("The Other Side of the Tracks"). Then she meets Noble Eggleston, from the "right" side of the tracks, and they feel an instant connection due to the presence of their "I Love You Theme" that plays whenever they touch. He invites her to his Sweet Sixteen party, and she agrees to go. At the party, Noble's friends all try to out-snob each other ("The Rich Kid Rag"). Noble's mother discovers that Belle lives in Drifter's Row and demands that Belle be sent away. Belle and Noble profess their love ("I Love You"), and Belle agrees to find wealth, culture, and social position to become equal to Noble so they can be together. She begins her journey, ignoring her friend George, who tells her it will only lead to her "getting kicked in the heart"("The Other Side of the Tracks (Reprise)").
Her first conquest is old, rich banker Mr. Pinchley, who is the "meanest, nastiest, and stingiest man in town" who refuses to help the poor. After Belle gets him to look at himself, Mr. Pinchley decides he no longer wants to be hated. Belle agrees to help him, and he wins the love of his son, and the citizens of Drifter's Row due to his generosity ("Deep Down Inside"). He asks Belle to marry him, and, as it will award her with wealth, culture, and social position, she accepts. When she gives him a hug, a gun accidentally goes off, killing him. Arrested and put on trial for murder, she meets Benny and Bernie Buchsbaum in jail and they advise her to become a star ("Be a Performer"). With Noble (who is now attending Harvard and Yale) helping her, she is acquitted and becomes a famous vaudeville performer with her own signature number ("Dimples").
Benny and Bernie fire her when they get a new star, and she is forced to become a camera girl at a club. There, she discovers Noble's mother is forcing him to marry Ramona, his wealthy friend. Devastated, she runs off. Then, the grand Val du Val makes his entrance and performs his sexually implicit song ("Boom-Boom") with his "Val du Val-ettes". Deciding she has nothing left to live for, she begins to jump off the roof. Val du Val notices this and prevents her from jumping, and leaves her in the hands of the club's owner. The owner is revealed to be George, who charms her ("I've Got Your Number") and gets her pregnant.
At a party during World War I she meets and marries poor-sighted soldier Fred Poitrine ("Real Live Girl"), but Fred dies in action from a serious digit wound. Belle has her baby and discovers that Noble is in trouble in France. She leaves the baby to her mother, quickly travels to France and, after failing to find Noble, stays to cheer up the troops ("Real Live Girl (Reprise)"). She then finds Val du Val has been left by his lover and now has amnesia. Belle uses the "booms" of the cannons to help him remember ("Boom-Boom (Reprise)"). His memory returns and they decide to marry. However, Noble has also arrived and Belle decides to marry him instead. When Val returns, she informs him of this, but he believes she "jilted" him like his old girlfriend and loses his memory again. Remembering how Val saved her life, she marries Val and tells Noble she no longer loves him ("Finale Act I: Boom-Boom").
Five years later, Belle and Val sail on the SS Gigantic. She runs into Noble again, and, with the help of their "I Love You Theme", they reveal they still love each other. The boat hits an iceberg and Noble helps save the passengers while they reaffirm their love ("I Love (Sinking) You (Reprise)"). Discovering Belle loves Noble, Val du Val believes he has been "jilted" and loses his memory. He dies when he forgets how to swim, allowing Belle to sue the boat company and become rich — attaining the wealth she had been seeking.
Bernie and Benny ask her to fund their next movie with her enormous fortune, and she accepts both the charity and the lead role ("Poor Little Hollywood Star"). While looking for a producer, she meets the director Otto Schnizler ("Be a Performer (Reprise)"). As they shoot the movie, Otto shows an actor what to do with a stage knife but actually stabs himself, and Belle laments ("Little Me"). However, she receives a huge award for her role in the movie – meaning she has attained culture.
During The Great Depression, Ramona's family has lost all of their money, and Miss Eggleston forces her and Noble to divorce, leaving him free for Belle.
Belle next journeys to Monte Carlo and meets Prince Cherney. He has a heart attack when he gambles his country's entire treasury and loses. He needs his hypodermic needle, but Belle gives him his fountain pen, leading him close to death. He and his many mourners make a huge deal out of it ("Goodbye (The Prince's Farewell)"), but Belle gives him part of her giant fortune to save his country. He discovers he is not going to die, and in gratitude he makes her "Countess Zoftig"—she has attained social position. She then reveals that she had given him wine that she poisoned so he wouldn't suffer. The Prince suddenly dies, but Belle now has wealth, culture, and social position and can return to Noble.
Noble, now Governor of both North and South Dakota, agrees to marry Belle, and, in celebration, she makes him drink. This causes him to become alcoholic, and he is impeached. Ashamed, he leaves Belle. Devastated, she returns to Baby and marries George, where they now live in The Bluff ("Here's to Us"). George had recently discovered Noble is a bum, and Noble has decided to clean up to return to Belle.
After Belle finishes her story for the autobiography, Baby announces she and Noble Junior (whom Noble had while with Romona) are getting married. Patrick Dennis announces he is done with his research, but right then Miss Eggleston arrives to kill Belle. Belle had pushed Noble to alcohol and herself to poverty, and she has decided to end it. Noble returns to claim Belle, but his mother has already shot. At first it appears Noble was shot, but then it is revealed that she shot George, who dies. Their "I Love You Theme" playing, Belle decides to marry Noble even though he no longer has wealth, culture, and social position and they all live happily ever after ("Finale Act II: Here's To Us"). (Or, in The 1999 Revival ("Finale Act II: I Love You").)