Catherine's technique to sell her clocks is to blackmail illegitimate couples such as Jacques and Gisèle. However when Gisèle's husband Pierre walks in on them, Catherine pretends to be Jacques's lover to save the day. Jacques then gets caught up in her schemes when the next couple she deals with turn out to be jewel thieves who kidnap them.

A person who had usurped the identity of a famous writer of detective stories was killed in the train. Why and by whom?