Meet Nestor (Jack Lemmon), a young man with a very complicated love-life. Employed as a business manager to Irma La Douce (Shirley MacLaine), a proud and profitable lady of the streets, the poor guy has gone and fallen in love with her! So how do you keep a popular Parisienne like Irma faithful? Simple. Disguise yourself as an elderly English Lord, who immediately becomes Irma's sole client and means of support! But what's a jealous manager to do when the illustrious Irma claims that the man she's really in love with is not the smitten Nestor, but the dotty old lord?