The Apartment (1960)

C.C. Baxter (Jack Lemmon) is an office clerk who curries favor with the executives in his office by giving them the key to his small apartment for the odd afternoon dalliance. Among them is his callous boss, J.D. Sheldrake (Fred MacMurray), who Baxter eventually learns is using his place to sleep with Miss Kubelik (Shirley MacLaine), the sweet elevator operator the clerk has loved from afar. When Sheldrake coldly dumps the vulnerable young woman, she tries to commit suicide, but is saved by the intervention of Baxter.