Fog can be a sudden thing on the Maine coast. On even the clearest mornings, swirling gray mists can sometimes appear in an instant, covering the earth with an opacity that makes it hard to see even one’s own feet on the ground. On this particular September morning it descended at 5:30, about the time Lucinda Cassidy and her companion Fritz, a small dog of indeterminate pedigree, arrived at the cemetery on Vaughan Street...