On a breezy Friday evening, Alain Coste finds himself in his office, preparing to head home. Just before he leaves, his personal assistant, Claire, manages to delay him long enough to stealthily snag his keys. Once Alain departs, she is quick to inform Cécile Coste that he is on his way out. Alain, oblivious to the unfolding drama, visits Georges Colinet, his doctor, who informs him that he must undergo surgery for his tennis elbow. Alain, however, is gripped by fear about the operation's risks, despite Georges' reassurances to the contrary.
Later, in a nearby park, Alain begins to record a series of last wills and testaments on a pocket tape recorder. He plans to divide his assets equally between his wife and children while designating the vast majority of his company to his kids. In a generous gesture, he also decides to donate some money from his Swiss account to the Fund for Cancer Research.
Back at home, Cécile is in a tizzy, worried that Alain hasn't returned yet, as this is a recurring fear of hers. As preparations for a surprise birthday party for Alain unfold, her daughter Louise is in the kitchen assisting her boyfriend Henry. Suddenly, their celebration is interrupted by the arrival of Police Inspector Pascal Manise who is dropping off his wife Agnès. The tension heightens when Dr. Colinet unexpectedly stops by, adding to Cécile's anxiety over Alain's whereabouts.
Meanwhile, lost in thought and in his own neighborhood, Alain teeters on the brink of a vehicular mishap with Manise. When he finally arrives home, Alain lies about his delay, claiming he gave Claire a ride. As the birthday festivities commence, Rémy and Louise perform "Les roses blanches" instead of the traditional "Happy Birthday to You." The seemingly innocent celebration is punctuated by Agnès wishing Alain, “may you have more days like this,” a comment he perceives as a dark omen.
In a moment alone, Cécile confronts Alain, sensing he is evading the truth about Claire. Alain fabricates a story about a car accident while seizing the moment when Agnès faints to further concoct a tale, using a hammer to damage his own Jaguar to validate his lie. Cécile, meanwhile, has called Manise regarding Agnès' condition, prompting his arrival to assist.
As the new day dawns, escalating tensions lead Cécile to enlist Manise to follow Alain, suspecting he harbors secrets. Manise cautions her that open communication with her husband might be wiser, but she is resolute in her decision. Alain undergoes a series of panic-stricken episodes and records his mounting health concerns, all while Cécile meticulously tracks his whereabouts through Claire's observations.
Their lives are intertwined in a web of suspicion and misunderstandings as Cécile grows increasingly convinced Alain is hiding something. Alain's encounters with various characters lead him deeper into confusion and paranoia, especially after Louise reveals her breakup due to infidelity. The mystery thickens as Manise discovers troubling news regarding Agnès, further complicating the domestic landscape.
In a frantic pursuit of the truth, Alain grapples with betrayal and delusion, culminating in a near breakdown that threatens to expose both his secrets and those of his loved ones. Amidst confrontations, mistaken identities, and a barrage of accusations, hope remains that the tangled web of lies might eventually unravel, bringing clarity and resolution for everyone involved.
