The expression 'le coup de berger' alludes to final chess moves, and several remarks are made about chess in the voiceover: Claire (Virginie Vitry) moves her 'fou' (the bishop), the position of the board is turned round when her husband Jean (Jacques Doniol-Valcroze) plays a very unexpected move, and the coup de grâce comes at the end when Claire is fooled by her own sister Solange (Anne Doat)
What am I talking about? Well, Claire is in an adulterous relationship with Claude (Jean-Claude Brialy), who gives her an expensive fur coat which she can't wear without seriously arousing her husband's suspicions. So she hatches a plan to pretend she's taken the taxi home, discovered a left luggage ticket on the car floor, and she'll then ask her husband to collect the surprise find. She has quite a job getting him to agree to this, but tells him it might be a valuable find.
But when Jean comes home the next day with the suitcase Claude has left at the left luggage office all that's in it is the skin of a rabbit. Claire is in a no-win situation, and doesn't know what to think. However, when Claire and Jean hold a party at their home Solange turns up wearing the coat: yes, Jean's having an affair with her.
My reaction to this is that the film doesn't quite work because Claire could easily have collected the coat herself because of her husband's obvious initial lack of interest in taking the ticket. All the same, this is a fascinating early curiosity by Rivette, which was also written and produced by Chabrol.