An evil Lolita-babysitter terrorizes a pregnant mother of two--in a tinkertoy suspense tale that requires the pregnant heroine to spend most of her time being thoroughly helpless and incredibly dumb. This dithering idiot is Manhattan's young, miscarriage-prone Mrs. Mary Shelburne, who needs a girl to help care for the kids during a summer-house vacation. So when 14-year-old Cynthia Ricks appears looking dumpy and demure, Mary hires her pronto. Little does Mary know, however, that Cinni is a psycho/thief/nymph who has already killed the mongoloid baby of her married lover. And once at the rented Long Island beach house, Cinni goes into action--revealing her well-developed bod, wooing the kids away from Mary, and seducing Mary's highly seduceable, weekend-visiting husband. But even when Mary realizes what gives (""She had a temptress in her home. A Lorelei. . .""), she's apparently unable to enlist any help from friends and neighbors. She finally does fire Cinni (after escaping from a murder attempt), but bungles it badly and allows Cinni to kidnap the kids; Cinni, thwarted in her plan to get pregnant by Mary's hubby, intends to drown the little ones. At the climax, Mary suddenly turns into a swimming Superwoman, but it's an unconvincing metamorphosis. As always (remember Charlotte Armstrong's Mischief?), the evil-babysitter routine provides a basic scare quotient, but Crane's clumsy treatment soon turns the initial chill into whiny foolishness.