They decide to both rent out the apartment. And use it exclusively for where they can have their secret sexual trysts. But with this twist. They decide they don’t want to know anything about the other person — not their names, not their jobs, not their family life. Nothing personal. The apartment will be a haven from their real lives, and as sort of a sanctuary for expressing their sexuality. Pure animal passion with no romance and no relationship aside from the sex.
Things are going great for most of the movie. And they’re able to work out all the kinks of their sexuality. Sort of the notion of sex as a pure self-indulgence with no other strings attached.
Until the Marlon Brando character makes the mistake of falling in love with her. And he tells her his real name and what he does for a living and all the other details of his personal life.
So, of course, she takes out a gun and shoots him dead. The End.
Those French are so romantic.