Alain, an insurance broker, embarks on a journey to find a new home in the suburbs of France alongside his wife, Marie Duval. Their adventure begins when the estate agent, Franck Chaize, unveils a peculiar feature of one property: a tunnel located in the basement that possesses the extraordinary ability to reverse a person's age by three days for anyone who dares to enter it, while simultaneously fast-forwarding time by twelve hours. Intrigued by this astonishing discovery, Marie's enthusiasm convinces Alain to go ahead with the purchase of this unique house.
Upon moving in, Alain decides to host his boss, Gérard, and his girlfriend, Jeanne, for a home tour. During this visit, Marie becomes engrossed with the tunnel, frequently utilizing its age-defying properties. The dinner takes a surprising turn when Gérard, under pressure from Jeanne, discloses an intimate secret—he has undergone a surgery to receive an electronic penis, which he hilariously controls via his smartphone. However, not long after, Gérard has an accident at the shooting range that causes damage to his unique appendage. This unforeseen circumstance prompts him to seek repairs in Japan, where the transplant took place, and leaves Alain in charge of softening the blow to Jeanne by claiming he is on a business trip. Meanwhile, Jeanne, who works at a bookstore, begins to make assertive advances towards Alain, leaving him in a compromising position.
As time progresses, Alain expresses skepticism regarding the tunnel's effects on ageing. To probe its capabilities, Marie brings a rotten apple into the tunnel. Amazingly, the apple emerges shiny and fresh, but upon Alain tasting it, he discovers it is crawling with ants. A subsequent trip to Doctor Urgent reveals that the tunnel only affects skin—internally, individuals remain unchanged in age, leading to a bittersweet realization for the couple. Marie confides in Alain her aspirations to become a renowned model, which fuels her obsession with reclaiming her youth.
The narrative continues as Gérard calls upon Alain to purchase a birthday gift for Jeanne. In a fateful incident, Alain encounters Franck, who cautions him about the health risks associated with the tunnel, recommending its use in moderation to prevent severe physical and psychological repercussions. The tension escalates when Alain witnesses Jeanne engage in an affair with an employee at her bookstore.
After a prolonged stay in Japan due to complications with his surgery, Gérard ultimately returns to France, but not without complications; he comes back with a new girlfriend, Mimi, leading to a series of romantic flops. As Alain and Marie argue over the tunnel's implications, Alain grows increasingly desperate to close it down.
The film takes a dark turn when Marie, having fully embraced the tunnel's power, emerges transformed into a 19-year-old version of herself. Despite her newfound youth, she faces struggles breaking into the modeling industry. In an alarming sequence of events, Gérard's electronic penis malfunctions while he is driving, leading to a catastrophic car accident. With Marie suffering from a severe emotional breakdown, Alain ultimately decides to seal the tunnel for good. However, in a harrowing revelation while hospitalized, Marie resorts to self-harm, uncovering that her body is overrun with ants—a shocking symbol of her inner turmoil and the unforeseen consequences of her attempts at regaining youth.
