When Mary-Margaret O'Shea, a woman of a certain age with mildly Machiavellian tendencies, discovers a homicide scene and meets an unimpressive lead investigator, she realizes that she has no choice but to solve the crime herself. With little help from Michael, her police-detective son, she enlists Arthur, her eccentric housekeeper, to help her find the killer.

In Mary-Margaret and The Case of The Lapsed Parishioner, a series of assumptions and misguided steps may lead her to the killer, or they may make her the next victim.