Jane Hersey--18, pregnant, and spurned by sleazy Martin Moon, the happy father--gets her own back by stealing his blackmail file (including an incriminating letter written by her own father, a Congressional candidate) and running to Boston's Convent of Our Lady of Good Counsel, ``the only order in America to have a stockbroker.'' There, she's taken under the wing of Sister Cecile--heiress, nun, and licensed p.i. (no, really)--who, with the help of her pre-convent old flame and current lawyer Paul Dorys, whisks Jane off to a convent in Paris and throws dust in the eyes of Moon and his goons. But this is all a prelude to Paul's attempt to help another client, foulmouthed old Lyuba McVey, to avenge the killing of her no-good son Ray by drug- dealing trashman Howie Marshall, who's determined to get back at her for getting him demoted. Okay, there are problems with the double plot and with Sister Cecile, who comes on like a computer's idea of a high-concept shamus. But first-novelist Sullivan--1991 winner of the publisher's Best First Private Eye Novel Contest--delivers the goods with a smartly paced, alternately sweet and tough story that has all the moves of a gangbusting nun.