A Vow of Obedience

In her fourth appearance, Sister Joan of the order of the Daughters of Compassion returns to her Cornwall convent after surviving the dangers of a six-week retreat in a cave in Scotland, which were recounted in last year's Vow of Sanctity. Sister Joan finds the garroted corpse of a teenage girl, dressed in a wedding gown, in a one-room school on the moor. The following day another young woman, similarly dressed, is found murdered in the same manner; the postmortems determine both victims were virgins. Then Sister Hilaria, the mistress of postulants, is injured by a hit-and-run driver. While readjusting to the orderliness of daily convent life, the unsanctimonious Sister Joan, a 36-year-old former artist, aids the appealing Detective Sergeant Mill in his efforts to prevent the killer from striking again. Likely suspects include a mysterious figure seen on convent grounds; an ambitious new sergeant, just posted with his timid wife to Cornwall from Birmingham; a local man's cousin recently released from a mental institution where he had been sent for stalking young women. Sister Joan remains the most refreshing and likable of the genre's nun-sleuths even in this shallow, easily penetrated plot. (Apr.)