The Apartment (1960)
Bud, a lonely insurance drone, loans his conveniently located apartment to various company managers for their extramarital affairs in exchange for a fast track ticket to the top of the corporate ladder. The bad stuff hits the fan at the office party on Christmas Eve when he discovers the object of his own affection, Fran, is his boss' mistress, Fran discovers she is one of many from his gossiping secretary and overdoses on sleeping pills. There is a happy ending. We promise.