Agnès Le Roux, a determined young woman, returns to Nice in 1976 seeking a fresh start after her marriage ends in failure. Her mother, Renée Le Roux, a wealthy widow, is embroiled in fierce conflicts with other shareholders over control of the struggling Palais de la Méditerranée, a significant casino located on the French Riviera. One disastrous night, the casino suffers a staggering loss of five million francs, allegedly due to tampering by professional gamblers.

In her quest for independence, Agnès kicks off a new venture by opening a charming bookstore that specializes in African artifacts and Asian textiles. Meanwhile, Renée, with support from her lawyer and advisor Maurice Agnelet, navigates the complexities of business ownership with her daughter’s decisive vote tipping the scales in her favor. Nevertheless, their mother-daughter relationship grows tense, as Renée, direct and unyielding, denies Agnès her rightful inheritance from her late father. Adding to the intrigue, Maurice, who is separated from his wife and has a close bond with his young son, becomes infatuated with the spirited and strong-willed Agnès. The emotional complexity intensifies when one of Maurice’s current partners, Françoise, fearing that he has developed feelings for Agnès, pays her a visit, warning her about Maurice’s notorious charm and alarming reputation.

As Maurice opens up and shares glimpses of his life, Agnès finds herself irresistibly drawn to him, resulting in an intense and passionate affair. Despite Maurice’s ambitions of rising to the position of managing director at the casino, Renée elects a more seasoned manager, leading him to take revenge by helping Agnès acquire the three million francs her mother stubbornly withheld. This intrigue unfolds as Italian mafia boss Jean-Dominique Fratoni, Renée’s business adversary, purchases Agnès’ shares, enabling her to vote against her mother in the crucial board meeting that culminates in Renée losing her power and influence, unraveling her wealth. With Fratoni taking charge of the casino, it soon meets an unfortunate closure, causing waves of scandal as news of Agnès’ betrayal surfaces, leaving her wrestling with guilt, which Maurice attempts to soothe. Despite understanding his flaws, Agnès embarks on a journey of trust by opening a shared bank account with him in Vevey.

Filled with sorrow from the collapse of her mother’s fortunes, Renée eventually learns of her daughter’s betrayal through Mario, a loyal Italian chauffeur who tries to uplift her spirits. Initially, Agnès finds joy in her relationship with Maurice, yet her demands complicate things; her deepening love becomes burdensome, causing Maurice to distance himself. A critical moment occurs when Agnès surprises him at a meeting with his son, leading to a humiliating scene where she is forced to apologize for her feelings. Reaching a breaking point, Agnès tragically attempts to end her life by overdosing on pills. Renée rushes to the hospital, only for Agnès to reject her mother’s comforting presence. Soon after, Maurice takes control and moves Agnès back into her apartment but remains emotionally unresponsive as she spirals into despair. Eventually, Agnès vanishes without a trace, leaving Maurice to exploit her finances by transferring her assets into his account. Heartbroken, Renée embarks on a relentless two-decade pursuit to uncover the truth, suspecting that Maurice played a direct role in Agnès’ fate.

Years later, now elderly and frail, Renée’s effort pays off as the investigation into her daughter’s mysterious disappearance is reopened. Maurice, who has been living in Panama, returns to France voluntarily to confront the trial. Complicating matters, Françoise testifies against him, retracting an earlier alibi and leaving Maurice vulnerable in court. During this poignant trial, Renée passionately advocates for justice on behalf of her daughter. However, despite the tense proceedings, Maurice triumphantly secures an acquittal. The story concludes with a title card revealing that in 2014, prompted by his son’s testimony, Maurice was ultimately found guilty of Agnès’ death and sentenced to two decades behind bars.