This is the story of twins, one of whom is born with a rare neurological disorder that makes her unable to feel pain. The pain-free twin grows up with a sense of invulnerability and superiority over mere fearful mortals, while the "normal" twin, who is forced to monitor her sister's well-being since she could be seriously injured without being aware of it, has an overdeveloped sense of responsibility. It was a little disappointing in that the ethical and psychological ramifications were not explored as deeply as they could have been. But it was quite a page-turner, and having myself a slightly older sister with an overdeveloped sense of responsibility, I found enough food for thought to make it worthwhile reading. ( )