Terror of the Cabinet Noir

After dropping off one-time companion Hattie in the Twist, the Doctor finds himself in Paris, France in the 17th century during the reign of King Louis XIV. However, something is seriously wrong: Cardinal Richelieu still has the city in his iron grip when he should've died fifty years ago, but he is doing more than cheating death. He has allies that are not from around here and is planning to plunge France and the entire world into darkness. Will the Doctor and his new companion Julie be able to stop him or will they be killed by the "darkness"?