The latest of St. George's neatly manipulated mysteries is set on Cape Cod, where 16-year-old Matt has moved reluctantly from Colorado. Matt has not made friends at school and he resents the environmentalist chiding of classmate Julie's John Muir Club friends, who are concerned about Matt's walking his elderly neighbor's dog in the marsh. He's also in wrong with the police, and with surly new neighbor Mr. Vossert--for when Vossert arrives keyless at the cottage he's rented, Matt sees him trying to gain entry and reports a break-in. Walking the dog subsequently, Matt watches the Vosserts--and sometimes sees Mr. Vossert watching him with binoculars. Mrs. Vossert disappears for weeks, then reappears--but is it the same Mrs. Vossert? One morning, with the drapes wide open, Matt sees Vossert strangle his wife in the living room--but when the police arrive Mrs. Vossert is fine, and once more Matt looks bad. Finally, enlisting Julie's help, Matt gets a chance to break into the Vosserts' cottage and look for evidence--of what, he's not sure. Inevitably, though, he finds proof that Vossert has indeed killed and replaced his wife, and just as inevitably he is confronted by Vossert and mortally endangered in the process. But St. John is good at accomplishing the inevitable when it's least expected. Though the ecology interest is just tacked on and the romance interest that develops with Julie isn't much more rounded, it all adds up to a proficiently handled, sufficiently fleshed-out mystery.