Magnificent Sinner (original French title: Katia) is a 1959 French drama historical film by director Robert Siodmak about the romance between Tsar Alexander II of Russia and the then-schoolgirl Catherine Dolgorukov, who later became his mistress and finally his morganatic wife. It stars Romy Schneider as Katia, a schoolgirl who becomes the Tsar's mistress and Curd Jürgens as Tsar Alexander II of Russia. The film, originally released as Katia, was a remake of a 1938 French film of the same name, which starred Danielle Darrieux.