David Niven portrays the character from Baroness Orczy’s story in a stylish, though somewhat insubstantial, adaptation of "The Scarlet Pimpernel." The narrative follows his efforts to rescue French aristocrats facing danger during the Reign of Terror, prompting authorities to assign Cyril Cusack's character, “Chauvelin,” with finding him. Chauvelin uses pressure against the alluring new wife of Sir Percy Blakeney, “L…