Act 1
At a train station in Pau, France in 1964, 34-year-old Alex Dillingham reflects on his love life over the past 17 years ("Love Changes Everything"). A woman (Giulietta Trapani) replies to him that "it's all in the past."

Flashing to 1947, Rose Vibert, a 20-year-old actress, complains that their production of The Master Builder is a flop. The producer, Marcel, tries to placate Rose by introducing her to a fan, 17-year-old Alex ("A Small Theatre in Montpellier"). Alex and Rose have a brief tryst at his uncle George's villa in Pau ("Seeing is Believing", "The House in Pau"). George Dillingham, in Paris with his mistress, Giulietta Trapani, an Italian sculptor, returns to his villa to see for himself what Alex and Rose are doing ("An Art Exhibition in Paris", "A Memory of a Happy Moment"). Rose is attracted to George, who is overcome when he sees Rose dressed in a gown belonging to his beloved late wife, Delia, also an actress, and remarks how much Rose resembles Delia ("In Many Rooms in the House at Pau").

He advises Alex that all good things have to end, and that his interlude with Rose will be a memory. Alex insists that his relationship with Rose is real love ("On the Terrace"). George leaves, and Rose leaves to rejoin Marcel. Alex realizes that Rose had never taken him seriously ("At the House at Pau").

Two years later, Alex, now a soldier, visits his uncle in Paris, and is shocked to find that Rose is now George's mistress. He accuses her of chasing his uncle's money, but Rose protests that she really loves George. She admits that she did love Alex once, and the two, drawn to each other again, fall into bed ("George's Flat in Paris").

The next morning, an agitated Rose tells Alex to leave before George returns. Alex, enraged, pulls out his gun. Rose throws a candlestick at Alex, and the gun goes off, shooting Rose in the arm, and she faints ("First Orchestral Interlude"). After George arrives, he and Alex each try to convince the other that the other is the right man for Rose. George wins the debate, insisting that Alex should begin a new life with Rose, and Alex finally agrees ("She'd Be Far Better Off With You"). George then heads to Venice to see his former mistress, Giulietta. However, Rose orders Alex to leave, having chosen to stay with George. Alex leaves, and Rose and Marcel follow George to Venice. Rose intends to confront Giulietta and reclaim George. In Venice, Giulietta and Rose bond while discussing George's foibles. They both express surprise that the other woman is not at all what they'd imagined. George returns and says that he has lost most of his money ("Stop. Wait. Please").

Rose then asks George to marry her and he agrees. At the wedding, Giulietta shocks everyone by claiming her best man's rights and kissing Rose on the mouth. George, however, is delighted ("A Registry Office").

At "A Military Camp in Malaya", Alex receives a letter from Rose telling him that she married George, and they are expecting a child.

Act 2
Twelve years later at a theatre in Paris, Rose has risen to stardom and has a young lover, Hugo. Marcel and the rest of the cast celebrate the latest hit (A Month in the Country) ("Leading Lady"), but Rose insists that she must return to the villa at Pau and to her husband George and their 12-year-old daughter, Jenny. Marcel reintroduces her to the 32-year-old Alex ("At the Stage Door"). Rose is delighted and insists that he come with her to Pau. At the villa, Jenny is excited by the prospect of her mother's return. George is happy to see Alex, returning with Rose, and Jenny, who has heard much about him, meets him for the first time. Rose and George insist that Alex should stay with them ("Other Pleasures"). Meanwhile, in Venice, George's former mistress, Giulietta, ponders the meaning of stable, long-lasting love versus romantic infatuation ("There Is More to Love").

Two years later, Alex suggests that Jenny needs a Paris education, which upsets Rose, who suspects that her daughter has developed an unhealthy crush on Alex ("The Garden at Pau (Version 2)"). That evening Jenny appears wearing Delia's gown, much as Rose did ("On the Terrace (Version 2)"); George happily dances with his daughter. Jenny tries to draw Alex into the dance, but Alex politely refuses ("The First Man You Remember"). Later, Jenny and Alex are left alone, and Jenny finally convinces Alex to give her the last dance. Rose catches Jenny clasping Alex in a very adult fashion, and Alex leaves. Jenny tells her mother that Alex is the first to make her feel like a woman. Rose confronts Alex, who admits to having feelings for Jenny, but insists that he would never harm her ("The Vineyard at Pau"). Later, Jenny tells Alex that she loves him. She begs him to be honest, then kisses him ("Up in the Pyrenees").

George plans his wake, insisting that there should be dancing and fun. Rose tells him that he's bound to outlive them all ("George's Study at Pau"). At a circus in Paris, George, Rose, Alex, and Jenny are celebrating Jenny's fifteenth birthday ("Journey of a Lifetime"). George becomes agitated as he watches Jenny talking with Alex ("Falling"). Later, Alex puts Jenny to bed. Jenny tries to convince him that she's really in love with him, but Alex insists that they're just cousins. Jenny falls asleep, and Alex reflects that he knows he must not love her, but cannot help loving her. George overhears Alex and is enraged, suspecting the worst. He collapses, and Alex comes out of Jenny's room to find him dead ("Jenny's Bedroom").

At George's wake, Giulietta gives a eulogy celebrating George's unconventionality and his belief in living life to the fullest ("Hand Me the Wine and Dice"). Giulietta and Alex join in the dancing and are attracted to each other, eventually trysting in a hayloft. Jenny spies on them, while Marcel tries to comfort the grieving Rose. Alex, alone with Giulietta, wonders how to end his relationship kindly with Jenny. He returns to the villa for one last confrontation. Alex tries to explain to Jenny that their relationship was unnatural. She reminds him that he was only seventeen when he met Rose, and that she is no younger than Shakespeare's Juliet ("On the Terrace (Version 3)").

Rose bids Alex farewell, but then breaks down and begs Alex not to leave her. Alex, unsure of how to reply, leaves ("Anything But Lonely"). At the train station at Pau, as Alex and Giulietta wait for the train, Giulietta wonders what will happen when Jenny reaches legal majority in three years. Alex, unable to reply, reflects once more on how love changes everything ("It Won't be Long till Jenny's a Woman").