The Apartment (1960)

Lowly insurance clerk C.C. Baxter (Lemmon) tries to climb the corporate ladder by loaning his apartment out to executives having affairs. Problems arise, however, when he unwittingly falls for sweet elevator operator Fran Kubelik (MacLaine), the most recent girlfriend of his unfeeling boss J.D. Sheldrake (MacMurray). Highly acclaimed social satire.