The monster has always been the true subject of the Frankenstein story, and Kenneth Branagh's new retelling understands that. "Mary Shelley's Frankenstein" has all of the usual props of the Frankenstein films, brought to a fever pitch: The dark and stormy nights, the lightning bolts, the charnel houses of spare body parts, the laboratory where Victor Frankenstein stirs his steaming cauldron of life. But the center of the film, quieter and more thoughtful, contains the real story.