Middle-aged businessman Gerard Oury comes home early from a weekend trip hoping to surprise the wife. He hopes to take her out for a nice dinner. It is a surprise all right! As Oury pulls up the street he sees his wife, Jeanne Moreau, stepping from a sports car with a young man on her arm. The two engage in a bit of tongue wrestling before the man gets back in the car and drives off. Oury follows the car to the young man's apartment. He gets the man's name and address from the lobby and drives off. He grabs a hotel room and a bottle and proceeds to get plastered. He decides that rather than confront Moreau with her infidelity he will get even. He will get his revenge by making the lives of Moreau and her lover, Philippe Nicaud, miserable. He returns home the next day as planned and goes about his business as normal. Oury opens a post office box with the id from a former employee who is now deceased. He then sends an anonymous letter to his wife. The letter states that they know about her affair and for a sum of 50,000 francs they will not inform the husband. Moreau rushes to Nicaud and asks what they should do. Nicaud, a low rent piano player in various clubs has not a franc to his name. Nicaud borrows the money from a former lover and the cash is sent. Oury collects the cash and has a good laugh. Oury meanwhile has hired a private eye to get some compromising photos of Moreau and Nicaud. "For a divorce case" he tells the P.I. Since Moreau and Nicaud are still seeing each other the pictures are rather easy to come up with. Oury waits several days and sends another request for cash. He includes one of the photos and hints the others will be sent to her husband. That night around the dinner table Moreau asks Oury for some money. "I would like to get some new dresses" she says. "I can't have my wife dressed in anything but the best" and hands over a thick wad of cash. Of course he knows he will get it all back. Oury is quite enjoying himself as his plan unfolds like clockwork. The perfect plan is about to hit the first of several speed bumps. Moreau and Nicaud decide to hire some goons to watch the post office and see who comes for the package. Oury picks up the envelope and is quickly grabbed up by a pair of large gentlemen and taken for a ride. Oury simply laughs and offers the pair 5 times their fee to switch teams. Money is money and the two report back to Moreau and company that they must have missed the blackmailer. Now Oury hires the private eye again to pretend he is a music agent. He is to offer Nicaud a well paying gig up the coast in Brussels. At the same time he sends the wife another set of photos and a demand for 500,000 francs. Moreau pawns her jewels and furs and sends the cash. Oury now uses the cash to have the P.I. give Nicaud a nice cash advance on his "upcoming" gig. All the while Moreau is keeping up a brave front at home. Oury is amazed that she has not confessed all. Nicaud calls Moreau over to his apartment to tell her the news about his new job. As he is packing Moreau sees the pile of "advance" cash sitting on his desk. She recognizes that the serial numbers are the same as the ones she just paid the blackmailer with! She realizes that her lover must really be the blackmailer! "The swine has been playing me all along." She pulls out a small gun from the desk and blasts Nicaud. Moreau goes home and confesses all to Oury, the affair, the blackmail and the killing. Oury, a quick thinker, asks where the body is and had anybody seen her there. "No." Oury grabs his car and goes over to Nicaud's apartment where he gathers up any evidence left by his wife. He then waits for nightfall, wraps the body in a carpet and loads the body in the car trunk. He takes said stiff to his company warehouse. There, he places the corpse at the bottom of a new wall that just happens be under construction. He mixes up a batch of concrete and adds a layer of several feet over the corpse. Now he returns home and tells the wife all is taken care of. Moreau of course now becomes the ever so devoted wife and all is great for Oruy. For a few months all is wonderful till Moreau finds the fake identification Oury used for the postal box. Moreau is floored! She has killed her one true love and it was all a plan of her husband. She writes a note to the police giving details that now clearly implicate Oury as Nicaud's killer. She sends off the note with her maid to be delivered. Moreau then takes Oury's own pistol and blows her brains out. Needless to say Oury is soon grabbed up for the murder of Nicaud. Neat little film with more than a few nice twists and turns. Those perfect plans do have a way of coming unglued!