As the film opens, it's 1954 and Sagan (played by Sylvie Testud), still in her teens, has become a literary celebrity overnight with the success of her first novel Bonjour Tristesse. Sagan is enjoying the high life with her close friends Jacques (Pierre Palmade) and Bernard (Lionel Abelanski), while spending her royalties on liquor, sports cars, partying and evenings at the casino. Sagan, a bisexual, enjoyed short-lived romances with lovers of both genders but in the Sixties she made an effort to settle down, marrying American expatriate Robert Westhoff (William Miller). While the two had a child, Westhoff's frequent homosexual affairs damaged their relationship beyond repair, and her longest-lasting romantic partnership was with Peggy (Jeanne Balibar), an editor at a fashion magazine. While Sagan continued to write, her appetite for alcohol and cocaine grew dramatically, and as her addiction became stronger, her literary output shrank and after years of failing to pay her taxes, she became dependent on the generosity of friends to survive and became a stranger to her only child.