Young cello student Ariane Chavasse eavesdrops on a conversation between her father, widowed private detective Claude Chavasse, and his client, "Monsieur X". After learning of his wife's daily trysts with American business magnate Frank Flannagan in Room 14 at the Paris Ritz, Monsieur X announces he will shoot Flannagan later that evening. Claude is nonchalant, regretting only the business he will lose (Flannagan is a well-known international playboy with a long history of numerous casual affairs). When Ariane cannot get the Ritz to put her through to Flannagan on the phone, and the police decline to intervene until after a crime has been committed, she decides to warn him herself.