Arletty (Laetitia Casta) says, in one of her brilliant oneliners: 'je suis comme tous les français, très occupée', with of course a pun on 'occupée (meaning 'busy' as well as 'occupied', as an allusion to the Nazi Occupation. At the end of the film, when the Resistance question her after the war, she says 'Mon cœur est français mais mon cul est international !' ('My heart is French but my ass is international!'.)
That last sentence just about sums up what this film is about: 'la môme de Courbevoie', the female actor Arletty's long and passionate, but also tumultuous, love affair with the German Luftwaffe officer Hans Jürgen Soehring (Ken Duken), a relationship which continued (if after the war from a distance) until Hans's death in 1960. There's an important amorous affair between Antoinette d'Harcourt (Marie-Josée Croze), for example, but Arletty is prepared to renounce that for Hans.
Many other famous characters (obviously appearing by actors) play a part in this, such as Sasha Guitry, Jacques Prévert, Pierre Brasseur, Marcel Carné and Jean-Louis Barrault, but obviously Arletty and Hans are the main characters.