Renowned for steamy melodramas, Lana Turner rarely starred in lavish costume dramas. But Diane was an exception that showcased her beauty and poise amid the finery of Renaissance France. Turner plays Diane de Poitiers in this historical romance, her last film under contract to MGM. The story begins in the mid-1500s. A marriage between France's Prince Henri (Roger Moore) and Italy's Catherine de Medici (Marisa Pavan) has the potential to establish peace, so Diane is asked to tutor Henri in the ways of a courtier. Fencing and etiquette are part of the instruction; falling in love is not. Diane and Henri begin an affair that continues despite the royal union...and precipitates a Medici plot to wrest control of France.