Any Wednesday (1966)

Well-to-do Manhattan businessman John Cleves (Jason Robards) keeps his mistress, Ellen Gordon (Jane Fonda), in an apartment on the Upper East Side. His new secretary, Miss Linsley (Ann Prentiss), knows about the apartment but not about Ellen, and mistakenly sends both new out-of-town client Cass Henderson (Dean Jones) and the suspicious Mrs. Cleves (Rosemary Murphy) to the address. The resulting tangle of mistaken identities puts all four into a series of awkward situations.