In 2010, disillusioned screenwriter Gil Pender and his fiancée Inez vacation in Paris with Inez's wealthy parents. Gil, struggling to finish his debut novel about a man who works in a nostalgia shop, finds himself drawn to the artistic history of Paris, especially the Lost Generation of the 1920s, and has ambitions to move there, which Inez dismisses. By chance, they meet Inez's friend Paul and his wife Carol. Paul speaks with great authority but questionable accuracy on French history, annoying Gil but impressing Inez.
Intoxicated after a night of wine tasting, Gil decides to walk back to their hotel, while Inez goes with Paul and Carol by taxi. At midnight, a 1920s car pulls up beside Gil and delivers him to a party for Jean Cocteau, attended by other people of the 1920s Paris art scene. Zelda Fitzgerald, bored, encourages her husband Scott and Gil to leave with her. They head to a cafe where they run into Ernest Hemingway and Juan Belmonte. After Zelda and Scott leave, Gil and Hemingway discuss writing, and Hemingway offers to show Gil's novel to Gertrude Stein. As Gil leaves to fetch his manuscript, he returns to 2010; the cafe is now a laundromat.
The next night, Gil tries to repeat the experience with Inez, but she leaves before midnight. Returning to the 1920s, Gil accompanies Hemingway to visit Gertrude Stein, who critiques Pablo Picasso’s new painting of his lover Adriana. Gil becomes drawn to Adriana, a costume designer who also had affairs with Amedeo Modigliani and Georges Braque. Adriana reads the first line of Gil’s novel, praises it, and admits she has always had a longing for the past.
Gil continues to time travel the following nights. Inez grows jaded with Paris and Gil's constant disappearing, while her father grows suspicious and hires a private detective to follow him. Adriana leaves Picasso and continues to bond with Gil, who is conflicted by his attraction to her. Gil explains his situation to Salvador Dalí, Man Ray, and Luis Buñuel; as surrealists, they do not question his claim of coming from the future. Gil later suggests the plot of The Exterminating Angel to Buñuel.
While Inez and her parents travel to Mont Saint Michel, Gil meets Gabrielle, an antique dealer and fellow admirer of the Lost Generation. He later finds Adriana's diary at a book stall, which reveals that she was in love with Gil and dreamed of being gifted earrings before making love to him. To seduce Adriana, Gil tries to steal a pair of Inez's earrings, but is thwarted by her early return to the hotel room.
Gil buys new earrings and returns to the past. After he gives Adriana the earrings, a horse-drawn carriage arrives, transporting them to the Belle Époque, an era Adriana considers Paris's Golden Age, They go to the Moulin Rouge where they meet Henri de Toulouse-Lautrec, Paul Gauguin, and Edgar Degas, who all agree that Paris’ best era was the Renaissance. Adriana is offered a job designing ballet costumes; thrilled, she proposes to Gil that they stay, but he, observing the unhappiness of Adriana and the other artists, realizes that chasing nostalgia is fruitless because the present is always "a little unsatisfying." Adriana decides to stay, and they part ways.
Gil rewrites the first two chapters of his novel and retrieves his draft from Stein, who praises his rewrite but says that on reading the new chapters Hemingway did not believe that the protagonist does not realise that his fiancée, based on Inez, is having an affair with the character based on Paul. Gil returns to 2010 and confronts Inez, who admits to having slept with Paul, but disregards it as a meaningless fling. Gil breaks up with her and decides to move to Paris. The detective following him takes a "wrong turn" and ends up being chased by the palace guards of Louis XVI just before a revolution breaks out. While walking by the Seine at midnight, Gil encounters Gabrielle. As it begins to rain, he offers to walk her home, and learns that they share a love for Paris in the rain.
