Some retreat. Even the Smartts, the newlyweds who accompany Mother Lavinia Grey, pastor of St. Bede's Episcopal Church, for a weekend at St. Hugh's priory, spell trouble. (Their name should really be the Lustfulls.) And when she finds that the assembled flock includes her old nemesis Father Rupert Bingley and detestable lawyer/politico Rodman Sedgewick (whose wife thinks he's come to St. Hugh's for a tryst with Mother Grey), she can see how the Great Silence enjoined from dusk till dawn might be the most sociable time all weekend. When Sedgewick's old math teacher, Brother Basil, is killed with one of Delight van Buskirk's knitting needles, Mother Grey--enjoying her hardcover debut after three paperbacks--not only can't prevent the local law from finding out about Brother Christophe, a novice who's hiding in St. Hugh's illegally, but she can't even prevent him from being dragged away in cuffs as the number-one suspect. Still, she fights back--with plots against Sedgewick (``In my heart I know he's guilty,'' she says calmly), with questions about St. Hugh's antiques, and finally with perhaps the most offhand unmasking of a criminal in mystery fiction. Mild, well-bred work from Gallison (Jersey Monkey, 1992, etc.), most likely to appeal to readers whose pulses quicken at the prospect of a page-and-a-half chart on ``The Critical Half- Hour.''