Hélène Régnier's (Stéphane Audran) mentally ill husband Charles goes into such a rage that he accidentally injures their son Michel. As a result, Charles retreats back to his rich and manipulative parents, who blame Hélène for his condition and now want custody of Michel. They hire a family acquaintance, Paul Thomas (Jean-Pierre Cassel), who needs money to spy on Hélène and find a dark mark to use against her in court. A diabolical plan is born to ruin Hélène's life....