While awaiting the arrival of their parents from London, teenager Mark and his younger sister Caroline are snowed in at their newly rented home in Northern Wales. Strange noises in the house lead to their discovery of fresh tracks in the snow leading toward the barn where they find a tunnel under the floor through which a neighbor is running out stolen sheep. Mark and Caroline have uncovered a whole ring of sheep rustlers with whom they eventually tangle in the tradition of the stiff-upper-lip-uddle-through school of younger British detectives. The author's way with the winter Welsh landscape maintains atmosphere and her introduction of reticent, superstitious villagers lifts this a bit above the general run of mysteries involving this well-worked plot. Occasional black and white drawings by Velma Ilsey are excellent.