The film begins in Paris, somewhere around the year 1902. Claude Roc (Leaud), a young middle-class Frenchman, meets Ann Brown (Markham), a young Englishwoman, and they become quick friends. Ann invites Claude to spend the holidays at her family's mansion, where we meet Ann's widowed mother (Marriott) and younger sister Muriel (Tendeter). During the holidays, Claude, Ann and Muriel become very close and Claude gradually falls in love with Muriel. Not fully knowing the couple's intentions, both families lay down a one-year-long separation without any contact before agreeing to get married. Claude goes back to Paris when he has many love affairs, and eleven months later he sends Muriel a break-off letter. A despondent Muriel sinks into a deep depression, and upon returning to Paris to defend her sister, Ann falls for Claude. This instigates a love triangle that consumes the threesome for the next twenty years.
Source : Wikipedia