Frank is assisted by Sister Stephanie ("Steve") Oskowski, a nun assigned to the parish to minister to its members.

The humor largely emerges from the intrusions into this "family circle" by Father Phil, who, like an unwanted relation, disrupts the cosy informality of the household.

In the first season, Sister Steve lives in a convent with the other nuns in her Order ("The Pretty Baby Mystery"); but in the second season she takes her meals at the Rectory, and when her brother is accused of murder (in the episode "The Sanctuary Mystery") he's shown sheltering in her bedroom at the Rectory.

Between attending to parish secretarial and housekeeping duties, Marie takes phone messages for the duo while they're out travelling all over the city investigating their latest case.

In addition, Phil's attempts at keeping on the Bishop's good side (another running gag) are a further source of comic relief.

In the episode The Legacy Mystery (aired April 4, 1990), Tom Bosley was reunited with his Happy Days co-star, Marion Ross, who plays the mother of a wealthy businessman accused of stealing valuable artwork.

There were few similarities to the books written by McInerny, with the principal character even changing his name (in the novels, he was Roger Dowling), while the setting was changed from a rural parish in the small town of Fox River to the city of Chicago – a setting which was borrowed from another series of novels by McInerny, concerning a Chicago nun.

For TV, the nun changed her name, became streetwise, lost many years of age, and metamorphosed into Father Dowling's youthful assistant.

In the first episode of the series, "The Missing Body Mystery", he is brought in by the Bishop to evaluate Father Dowling's parish, and Prestwick is initially appointed to replace him.

When ABC picked up the show for the second and third seasons (initially as a mid-season replacement), the show's executive producer, Dean Hargrove, together with Joyce Burditt, became the backbone of the writing team, frequently writing episodes jointly (Hargrove had begun his career as a scriptwriter, on the TV series The Man From U.N.C.L.E.).

For the third and longest season (the show's only full season), Brian Clemens, who had been producer and executive producer on The Avengers and had written many of its best regarded episodes, was added as a script consultant, and wrote or co-wrote many third-season episodes, a number of them in collaboration with co-producer Gerry Conway.

[4] At the time the show was being made, Bosley's co-star Tracy Nelson ("Sister Steve") was married to actor William R. Moses, the featured guest star who plays her ex-boyfriend in season one ("The Man Who Came to Dinner Mystery").

The program featured many notable guest stars, some before they had achieved fame, including: John Astin, Jack Bannon, Roscoe Lee Browne, Steven Culp, Stacy Edwards, Fionnula Flanagan, Michelle Forbes, Kurt Fuller, Annie Golden, Grant Heslov, Laurie Holden, Stanley Kamel, Andreas Katsulas, Yaphet Kotto, Diane Ladd, Anthony LaPaglia, Scott Marlowe, Kiel Martin, David McCallum, Colm Meaney, William R. Moses, Craig Richard Nelson, Leslie Nielsen, Ethan Phillips, John Slattery, Brenda Strong, Robin Thomas, Sada Thompson, Tony Todd, John Vernon, Kate Vernon, Robert Walden and David Warner.